<s>
An	O
XML	B-Language
schema	I-Language
is	O
a	O
description	O
of	O
a	O
type	O
of	O
XML	B-Protocol
document	I-Protocol
,	O
typically	O
expressed	O
in	O
terms	O
of	O
constraints	O
on	O
the	O
structure	O
and	O
content	O
of	O
documents	O
of	O
that	O
type	O
,	O
above	O
and	O
beyond	O
the	O
basic	O
syntactical	O
constraints	O
imposed	O
by	O
XML	B-Protocol
itself	O
.	O
</s>
<s>
These	O
constraints	O
are	O
generally	O
expressed	O
using	O
some	O
combination	O
of	O
grammatical	O
rules	O
governing	O
the	O
order	O
of	O
elements	O
,	O
Boolean	B-Algorithm
predicates	I-Algorithm
that	O
the	O
content	O
must	O
satisfy	O
,	O
data	O
types	O
governing	O
the	O
content	O
of	O
elements	O
and	O
attributes	O
,	O
and	O
more	O
specialized	O
rules	O
such	O
as	O
uniqueness	O
and	O
referential	B-General_Concept
integrity	I-General_Concept
constraints	O
.	O
</s>
<s>
There	O
are	O
languages	O
developed	O
specifically	O
to	O
express	O
XML	B-Language
schemas	I-Language
.	O
</s>
<s>
The	O
document	B-Language
type	I-Language
definition	I-Language
(	O
DTD	O
)	O
language	O
,	O
which	O
is	O
native	O
to	O
the	O
XML	B-Protocol
specification	I-Protocol
,	O
is	O
a	O
schema	O
language	O
that	O
is	O
of	O
relatively	O
limited	O
capability	O
,	O
but	O
that	O
also	O
has	O
other	O
uses	O
in	O
XML	B-Protocol
aside	O
from	O
the	O
expression	O
of	O
schemas	O
.	O
</s>
<s>
Two	O
more	O
expressive	O
XML	B-Language
schema	I-Language
languages	O
in	O
widespread	O
use	O
are	O
XML	B-Language
Schema	I-Language
(	O
with	O
a	O
capital	O
S	O
)	O
and	O
RELAX	B-Language
NG	I-Language
.	O
</s>
<s>
The	O
mechanism	O
for	O
associating	O
an	O
XML	B-Protocol
document	I-Protocol
with	O
a	O
schema	O
varies	O
according	O
to	O
the	O
schema	O
language	O
.	O
</s>
<s>
The	O
association	O
may	O
be	O
achieved	O
via	O
markup	O
within	O
the	O
XML	B-Protocol
document	I-Protocol
itself	O
,	O
or	O
via	O
some	O
external	O
means	O
.	O
</s>
<s>
The	O
XML	B-Language
Schema	I-Language
Definition	O
is	O
commonly	O
referred	O
to	O
as	O
XSD	O
.	O
</s>
<s>
The	O
process	O
of	O
checking	O
to	O
see	O
if	O
a	O
XML	B-Protocol
document	I-Protocol
conforms	O
to	O
a	O
schema	O
is	O
called	O
validation	B-Language
,	O
which	O
is	O
separate	O
from	O
XML	B-Protocol
's	O
core	O
concept	O
of	O
syntactic	O
well-formedness	B-Language
.	O
</s>
<s>
All	O
XML	B-Protocol
documents	I-Protocol
must	O
be	O
well-formed	O
,	O
but	O
it	O
is	O
not	O
required	O
that	O
a	O
document	O
be	O
valid	O
unless	O
the	O
XML	B-Protocol
parser	I-Protocol
is	O
"	O
validating	O
"	O
,	O
in	O
which	O
case	O
the	O
document	O
is	O
also	O
checked	O
for	O
conformance	O
with	O
its	O
associated	O
schema	O
.	O
</s>
<s>
DTD-validating	O
parsers	B-Language
are	O
most	O
common	O
,	O
but	O
some	O
support	O
XML	B-Language
Schema	I-Language
or	O
RELAX	B-Language
NG	I-Language
as	O
well	O
.	O
</s>
<s>
Validation	B-Language
of	O
an	O
instance	O
document	O
against	O
a	O
schema	O
can	O
be	O
regarded	O
as	O
a	O
conceptually	O
separate	O
operation	O
from	O
XML	B-Protocol
parsing	B-Language
.	O
</s>
<s>
In	O
practice	O
,	O
however	O
,	O
many	O
schema	O
validators	O
are	O
integrated	O
with	O
an	O
XML	B-Protocol
parser	I-Protocol
.	O
</s>
<s>
There	O
are	O
several	O
different	O
languages	O
available	O
for	O
specifying	O
an	O
XML	B-Language
schema	I-Language
.	O
</s>
<s>
The	O
primary	O
purpose	O
of	O
a	O
schema	O
language	O
is	O
to	O
specify	O
what	O
the	O
structure	O
of	O
an	O
XML	B-Protocol
document	I-Protocol
can	O
be	O
.	O
</s>
<s>
There	O
are	O
historic	O
and	O
current	O
XML	B-Language
schema	I-Language
languages	O
:	O
</s>
<s>
LanguageAbbrev.VersionsAuthority	O
Constraint	B-Language
Language	I-Language
in	I-Language
XML	I-Language
CLiX	B-Language
2005Independent	O
Document	O
Content	O
Description	O
facility	O
for	O
XML	B-Protocol
,	O
an	O
RDF	O
frameworkDCD	O
v1.0	O
(	O
1998	O
)	O
W3C	O
(	O
Note	O
)	O
Document	B-General_Concept
Definition	I-General_Concept
Markup	I-General_Concept
Language	I-General_Concept
DDML	B-General_Concept
v0	O
(	O
1999	O
)	O
W3C	O
(	O
Note	O
)	O
Document	B-Language
Structure	I-Language
Description	I-Language
DSD	O
2002	O
,	O
2005	O
BRICS	O
(	O
defunct	O
)	O
Document	B-Language
Type	I-Language
Definition	I-Language
DTD	O
1986	O
(	O
SGML	O
)	O
ISO	O
2008	O
(	O
XML	B-Protocol
)	O
ISO/IEC	O
Namespace	O
Routing	O
Language	O
NRL	O
2003	O
Independent	O
Namespace-based	O
Validation	B-Language
Dispatching	O
Language	O
NVDL	O
2006	O
ISO/IEC	O
Content	O
Assembly	O
Mechanism	O
CAM	O
2007	O
OASIS	O
REgular	B-Language
LAnguage	I-Language
for	I-Language
XML	I-Language
Next	I-Language
Generation	I-Language
RELAX	B-Language
NG	I-Language
,	O
RelaxNG	B-Language
2001	O
,	O
Compact	O
Syntax	O
(	O
2002	O
)	O
OASIS	O
v1	O
(	O
2003	O
)	O
,	O
v1	O
Compact	O
Syntax	O
(	O
2006	O
)	O
,	O
v2	O
(	O
2008	O
)	O
ISO/IEC	O
Schema	B-Language
for	I-Language
Object-Oriented	I-Language
XML	I-Language
SOX	O
?	O
</s>
<s>
Schematron	B-Language
N/A	O
2006	O
,	O
2010	O
,	O
2016	O
,	O
2020	O
ISO/IEC	O
XML-Data	B-Language
Reduced	I-Language
XDR	O
?	O
</s>
<s>
ASN.1	B-Protocol
XML	B-Protocol
Encoding	I-Protocol
Rules	I-Protocol
XER	O
?	O
</s>
<s>
The	O
main	O
ones	O
(	O
see	O
also	O
the	O
ISO	B-Language
19757	I-Language
's	I-Language
endorsed	I-Language
languages	I-Language
)	O
are	O
described	O
below	O
.	O
</s>
<s>
Though	O
there	O
are	O
a	O
number	O
of	O
schema	O
languages	O
available	O
,	O
the	O
primary	O
three	O
languages	O
are	O
Document	B-Language
Type	I-Language
Definitions	I-Language
,	O
W3C	O
XML	B-Language
Schema	I-Language
,	O
and	O
RELAX	B-Language
NG	I-Language
.	O
</s>
<s>
DTDs	O
are	O
perhaps	O
the	O
most	O
widely	O
supported	O
schema	O
language	O
for	O
XML	B-Protocol
.	O
</s>
<s>
Because	O
DTDs	O
are	O
one	O
of	O
the	O
earliest	O
schema	O
languages	O
for	O
XML	B-Protocol
,	O
defined	O
before	O
XML	B-Protocol
even	O
had	O
namespace	O
support	O
,	O
they	O
are	O
widely	O
supported	O
.	O
</s>
<s>
Internal	O
DTDs	O
are	O
often	O
supported	O
in	O
XML	B-Protocol
processors	O
;	O
external	O
DTDs	O
are	O
less	O
often	O
supported	O
,	O
but	O
only	O
slightly	O
.	O
</s>
<s>
Most	O
large	O
XML	B-Protocol
parsers	I-Protocol
,	O
ones	O
that	O
support	O
multiple	O
XML	B-Protocol
technologies	O
,	O
will	O
provide	O
support	O
for	O
DTDs	O
as	O
well	O
.	O
</s>
<s>
Facilities	O
for	O
defining	O
uniqueness	O
constraints	O
and	O
referential	B-General_Concept
integrity	I-General_Concept
are	O
more	O
powerful	O
:	O
unlike	O
the	O
ID	O
and	O
IDREF	O
constraints	O
in	O
DTDs	O
,	O
they	O
can	O
be	O
scoped	O
to	O
any	O
part	O
of	O
a	O
document	O
,	O
can	O
be	O
of	O
any	O
data	O
type	O
,	O
can	O
apply	O
to	O
element	O
as	O
well	O
as	O
attribute	O
content	O
,	O
and	O
can	O
be	O
multi-part	O
(	O
for	O
example	O
the	O
combination	O
of	O
first	O
name	O
and	O
last	O
name	O
must	O
be	O
unique	O
)	O
.	O
</s>
<s>
XSD	O
1.1	O
adds	O
the	O
ability	O
to	O
define	O
arbitrary	O
assertions	O
(	O
using	O
XPath	B-Language
expressions	O
)	O
as	O
constraints	O
on	O
element	O
content	O
.	O
</s>
<s>
XSD	O
schemas	O
are	O
conventionally	O
written	O
as	O
XML	B-Protocol
documents	I-Protocol
,	O
so	O
familiar	O
editing	O
and	O
transformation	O
tools	O
can	O
be	O
used	O
.	O
</s>
<s>
As	O
well	O
as	O
validation	B-Language
,	O
XSD	O
allows	O
XML	B-Protocol
instances	O
to	O
be	O
annotated	O
with	O
type	O
information	O
(	O
the	O
Post-Schema-Validation	O
Infoset	O
(	O
PSVI	O
)	O
)	O
which	O
is	O
designed	O
to	O
make	O
manipulation	O
of	O
the	O
XML	B-Protocol
instance	O
easier	O
in	O
application	O
programs	O
.	O
</s>
<s>
This	O
may	O
be	O
by	O
mapping	O
the	O
XSD-defined	O
types	O
to	O
types	O
in	O
a	O
programming	O
language	O
such	O
as	O
Java	B-Language
(	O
"	O
data	O
binding	O
"	O
)	O
or	O
by	O
enriching	O
the	O
type	O
system	O
of	O
XML	B-Protocol
processing	O
languages	O
such	O
as	O
XSLT	B-Application
and	O
XQuery	O
(	O
known	O
as	O
"	O
schema-awareness	O
"	O
)	O
.	O
</s>
<s>
RELAX	B-Language
NG	I-Language
and	O
W3C	O
XML	B-Language
Schema	I-Language
allow	O
for	O
similar	O
mechanisms	O
of	O
specificity	O
.	O
</s>
<s>
And	O
both	O
of	O
them	O
are	O
,	O
or	O
can	O
be	O
,	O
defined	O
in	O
an	O
XML	B-Protocol
language	O
.	O
</s>
<s>
RELAX	B-Language
NG	I-Language
does	O
not	O
have	O
any	O
analog	O
to	O
PSVI	O
.	O
</s>
<s>
Unlike	O
W3C	O
XML	B-Language
Schema	I-Language
,	O
RELAX	B-Language
NG	I-Language
was	O
designed	O
so	O
that	O
validation	B-Language
and	O
augmentation	O
(	O
adding	O
type	O
information	O
and	O
default	O
values	O
)	O
are	O
separate	O
.	O
</s>
<s>
W3C	O
XML	B-Language
Schema	I-Language
has	O
a	O
formal	O
mechanism	O
for	O
attaching	O
a	O
schema	O
to	O
an	O
XML	B-Protocol
document	I-Protocol
,	O
while	O
RELAX	B-Language
NG	I-Language
intentionally	O
avoids	O
such	O
mechanisms	O
for	O
security	O
and	O
interoperability	O
reasons	O
.	O
</s>
<s>
RELAX	B-Language
NG	I-Language
has	O
no	O
ability	O
to	O
apply	O
default	O
attribute	O
data	O
to	O
an	O
element	O
's	O
list	O
of	O
attributes	O
(	O
i.e.	O
,	O
changing	O
the	O
XML	B-Protocol
info	O
set	O
)	O
,	O
while	O
W3C	O
XML	B-Language
Schema	I-Language
does	O
.	O
</s>
<s>
Again	O
,	O
this	O
design	O
is	O
intentional	O
and	O
is	O
to	O
separate	O
validation	B-Language
and	O
augmentation	O
.	O
</s>
<s>
W3C	O
XML	B-Language
Schema	I-Language
has	O
a	O
rich	O
"	O
simple	O
type	O
"	O
system	O
built-in	O
(	O
xs:number	O
,	O
xs:date	O
,	O
etc.	O
,	O
plus	O
derivation	O
of	O
custom	O
types	O
)	O
,	O
while	O
RELAX	B-Language
NG	I-Language
has	O
an	O
extremely	O
simplistic	O
one	O
because	O
it	O
is	O
meant	O
to	O
use	O
type	O
libraries	O
developed	O
independently	O
of	O
RELAX	B-Language
NG	I-Language
,	O
rather	O
than	O
grow	O
its	O
own	O
.	O
</s>
<s>
In	O
practice	O
it	O
is	O
common	O
for	O
a	O
RELAX	B-Language
NG	I-Language
schema	O
to	O
use	O
the	O
predefined	O
"	O
simple	O
types	O
"	O
and	O
"	O
restrictions	O
"	O
(	O
pattern	O
,	O
maxLength	O
,	O
etc	O
.	O
)	O
</s>
<s>
of	O
W3C	O
XML	B-Language
Schema	I-Language
.	O
</s>
<s>
In	O
W3C	O
XML	B-Language
Schema	I-Language
a	O
specific	O
number	O
or	O
range	O
of	O
repetitions	O
of	O
patterns	O
can	O
be	O
expressed	O
whereas	O
it	O
is	O
practically	O
not	O
possible	O
to	O
specify	O
at	O
all	O
in	O
RELAX	B-Language
NG	I-Language
(	O
<oneOrMore> or <zeroOrMore>	O
)	O
.	O
</s>
<s>
W3C	O
XML	B-Language
Schema	I-Language
is	O
complex	O
and	O
hard	O
to	O
learn	O
,	O
although	O
that	O
is	O
partially	O
because	O
it	O
tries	O
to	O
do	O
more	O
than	O
mere	O
validation	B-Language
(	O
see	O
PSVI	O
)	O
.	O
</s>
<s>
Although	O
being	O
written	O
in	O
XML	B-Protocol
is	O
an	O
advantage	O
,	O
it	O
is	O
also	O
a	O
disadvantage	O
in	O
some	O
ways	O
.	O
</s>
<s>
The	O
W3C	O
XML	B-Language
Schema	I-Language
language	O
,	O
in	O
particular	O
,	O
can	O
be	O
quite	O
verbose	O
,	O
while	O
a	O
DTD	O
can	O
be	O
terse	O
and	O
relatively	O
easily	O
editable	O
.	O
</s>
<s>
For	O
WXS	O
validators	O
that	O
will	O
follow	O
a	O
URI	B-Protocol
to	O
an	O
arbitrary	O
online	O
location	O
,	O
there	O
is	O
the	O
potential	O
for	O
reading	O
something	O
malicious	O
from	O
the	O
other	O
side	O
of	O
the	O
stream	O
.	O
</s>
<s>
W3C	O
XML	B-Language
Schema	I-Language
does	O
not	O
implement	O
most	O
of	O
the	O
DTD	O
ability	O
to	O
provide	O
data	O
elements	O
to	O
a	O
document	O
.	O
</s>
<s>
Although	O
W3C	O
XML	B-Language
Schema	I-Language
's	O
ability	O
to	O
add	O
default	O
attributes	O
to	O
elements	O
is	O
an	O
advantage	O
,	O
it	O
is	O
a	O
disadvantage	O
in	O
some	O
ways	O
as	O
well	O
.	O
</s>
<s>
It	O
means	O
that	O
an	O
XML	B-Protocol
file	O
may	O
not	O
be	O
usable	O
in	O
the	O
absence	O
of	O
its	O
schema	O
,	O
even	O
if	O
the	O
document	O
would	O
validate	O
against	O
that	O
schema	O
.	O
</s>
<s>
In	O
effect	O
,	O
all	O
users	O
of	O
such	O
an	O
XML	B-Protocol
document	I-Protocol
must	O
also	O
implement	O
the	O
W3C	O
XML	B-Language
Schema	I-Language
specification	O
,	O
thus	O
ruling	O
out	O
minimalist	O
or	O
older	O
XML	B-Protocol
parsers	I-Protocol
.	O
</s>
<s>
It	O
can	O
also	O
slow	O
down	O
the	O
processing	O
of	O
the	O
document	O
,	O
as	O
the	O
processor	O
must	O
potentially	O
download	O
and	O
process	O
a	O
second	O
XML	B-Protocol
file	O
(	O
the	O
schema	O
)	O
;	O
however	O
,	O
a	O
schema	O
would	O
normally	O
then	O
be	O
cached	O
,	O
so	O
the	O
cost	O
comes	O
only	O
on	O
the	O
first	O
use	O
.	O
</s>
<s>
WXS	O
support	O
exists	O
in	O
a	O
number	O
of	O
large	O
XML	B-Protocol
parsing	B-Language
packages	O
.	O
</s>
<s>
Xerces	B-Language
and	O
the	O
.NET	B-Application
Framework	I-Application
's	O
Base	O
Class	O
Library	O
both	O
provide	O
support	O
for	O
WXS	O
validation	B-Language
.	O
</s>
<s>
RELAX	B-Language
NG	I-Language
provides	O
for	O
most	O
of	O
the	O
advantages	O
that	O
W3C	O
XML	B-Language
Schema	I-Language
does	O
over	O
DTDs	O
.	O
</s>
<s>
While	O
the	O
language	O
of	O
RELAX	B-Language
NG	I-Language
can	O
be	O
written	O
in	O
XML	B-Protocol
,	O
it	O
also	O
has	O
an	O
equivalent	O
form	O
that	O
is	O
much	O
more	O
like	O
a	O
DTD	O
,	O
but	O
with	O
greater	O
specifying	O
power	O
.	O
</s>
<s>
Even	O
arbitrary	O
elements	O
specified	O
between	O
RELAX	B-Language
NG	I-Language
XML	B-Protocol
elements	O
can	O
be	O
converted	O
into	O
the	O
compact	O
form	O
.	O
</s>
<s>
RELAX	B-Language
NG	I-Language
provides	O
very	O
strong	O
support	O
for	O
unordered	O
content	O
.	O
</s>
<s>
RELAX	B-Language
NG	I-Language
also	O
allows	O
for	O
non-deterministic	O
content	O
models	O
.	O
</s>
<s>
What	O
this	O
means	O
is	O
that	O
RELAX	B-Language
NG	I-Language
allows	O
the	O
specification	O
of	O
a	O
sequence	O
like	O
the	O
following	O
:	O
</s>
<s>
RELAX	B-Language
NG	I-Language
allows	O
this	O
kind	O
of	O
specification	O
.	O
</s>
<s>
W3C	O
XML	B-Language
Schema	I-Language
requires	O
all	O
of	O
its	O
sequences	O
to	O
be	O
fully	O
deterministic	O
,	O
so	O
mechanisms	O
like	O
the	O
above	O
must	O
be	O
either	O
specified	O
in	O
a	O
different	O
way	O
or	O
omitted	O
altogether	O
.	O
</s>
<s>
RELAX	B-Language
NG	I-Language
allows	O
attributes	O
to	O
be	O
treated	O
as	O
elements	O
in	O
content	O
models	O
.	O
</s>
<s>
The	O
order	O
of	O
the	O
specification	O
of	O
attributes	O
in	O
RELAX	B-Language
NG	I-Language
has	O
no	O
meaning	O
,	O
so	O
this	O
block	O
need	O
not	O
be	O
the	O
first	O
block	O
in	O
the	O
element	O
definition	O
.	O
</s>
<s>
W3C	O
XML	B-Language
Schema	I-Language
cannot	O
specify	O
such	O
a	O
dependency	O
between	O
the	O
content	O
of	O
an	O
attribute	O
and	O
child	O
elements	O
.	O
</s>
<s>
RELAX	B-Language
NG	I-Language
's	O
specification	O
only	O
lists	O
two	O
built-in	O
types	O
(	O
string	O
and	O
token	O
)	O
,	O
but	O
it	O
allows	O
for	O
the	O
definition	O
of	O
many	O
more	O
.	O
</s>
<s>
Most	O
RELAX	B-Language
NG	I-Language
schemas	O
can	O
be	O
algorithmically	O
converted	O
into	O
W3C	O
XML	B-Language
Schemas	I-Language
and	O
even	O
DTDs	O
(	O
except	O
when	O
using	O
RELAX	B-Language
NG	I-Language
features	O
not	O
supported	O
by	O
those	O
languages	O
,	O
as	O
above	O
)	O
.	O
</s>
<s>
As	O
such	O
,	O
RELAX	B-Language
NG	I-Language
can	O
be	O
used	O
as	O
a	O
normative	O
version	O
of	O
the	O
schema	O
,	O
and	O
the	O
user	O
can	O
convert	O
it	O
to	O
other	O
forms	O
for	O
tools	O
that	O
do	O
not	O
support	O
RELAX	B-Language
NG	I-Language
.	O
</s>
<s>
Most	O
of	O
RELAX	B-Language
NG	I-Language
's	O
disadvantages	O
are	O
covered	O
under	O
the	O
section	O
on	O
W3C	O
XML	B-Language
Schema	I-Language
's	O
advantages	O
over	O
RELAX	B-Language
NG	I-Language
.	O
</s>
<s>
Though	O
RELAX	B-Language
NG	I-Language
's	O
ability	O
to	O
support	O
user-defined	O
data	O
types	O
is	O
useful	O
,	O
it	O
comes	O
at	O
the	O
disadvantage	O
of	O
only	O
having	O
two	O
data	O
types	O
that	O
the	O
user	O
can	O
rely	O
upon	O
.	O
</s>
<s>
Which	O
,	O
in	O
theory	O
,	O
means	O
that	O
using	O
a	O
RELAX	B-Language
NG	I-Language
schema	O
across	O
multiple	O
validators	O
requires	O
either	O
providing	O
those	O
user-defined	O
data	O
types	O
to	O
that	O
validator	O
or	O
using	O
only	O
the	O
two	O
basic	O
types	O
.	O
</s>
<s>
In	O
practice	O
,	O
however	O
,	O
most	O
RELAX	B-Language
NG	I-Language
processors	O
support	O
the	O
W3C	O
XML	B-Language
Schema	I-Language
set	O
of	O
data	O
types	O
.	O
</s>
<s>
Schematron	B-Language
is	O
a	O
fairly	O
unusual	O
schema	O
language	O
.	O
</s>
<s>
Unlike	O
the	O
main	O
three	O
,	O
it	O
defines	O
an	O
XML	B-Protocol
file	O
's	O
syntax	O
as	O
a	O
list	O
of	O
XPath-based	O
rules	O
.	O
</s>
<s>
Because	O
of	O
its	O
rule-based	O
nature	O
,	O
Schematron	B-Language
's	O
specificity	O
is	O
very	O
strong	O
.	O
</s>
<s>
It	O
can	O
even	O
specify	O
required	O
relationships	O
between	O
multiple	O
XML	B-Protocol
files	O
.	O
</s>
<s>
While	O
Schematron	B-Language
is	O
good	O
at	O
relational	O
constructs	O
,	O
its	O
ability	O
to	O
specify	O
the	O
basic	O
structure	O
of	O
a	O
document	O
,	O
that	O
is	O
,	O
which	O
elements	O
can	O
go	O
where	O
,	O
results	O
in	O
a	O
very	O
verbose	O
schema	O
.	O
</s>
<s>
The	O
typical	O
way	O
to	O
solve	O
this	O
is	O
to	O
combine	O
Schematron	B-Language
with	O
RELAX	B-Language
NG	I-Language
or	O
W3C	O
XML	B-Language
Schema	I-Language
.	O
</s>
<s>
This	O
allows	O
Schematron	B-Language
rules	O
to	O
specify	O
additional	O
constraints	O
to	O
the	O
structure	O
defined	O
by	O
W3C	O
XML	B-Language
Schema	I-Language
or	O
RELAX	B-Language
NG	I-Language
.	O
</s>
<s>
Schematron	B-Language
's	O
reference	O
implementation	O
is	O
actually	O
an	O
XSLT	B-Application
transformation	O
that	O
transforms	O
the	O
Schematron	B-Language
document	O
into	O
an	O
XSLT	B-Application
that	O
validates	O
the	O
XML	B-Protocol
file	O
.	O
</s>
<s>
As	O
such	O
,	O
Schematron	B-Language
's	O
potential	O
toolset	O
is	O
any	O
XSLT	B-Application
processor	I-Application
,	O
though	O
libxml2	B-Application
provides	O
an	O
implementation	O
that	O
does	O
not	O
require	O
XSLT	B-Application
.	O
</s>
<s>
Sun	O
Microsystems	O
's	O
Multiple	O
Schema	O
Validator	O
for	O
Java	B-Language
has	O
an	O
add-on	O
that	O
allows	O
it	O
to	O
validate	O
RELAX	B-Language
NG	I-Language
schemas	O
that	O
have	O
embedded	O
Schematron	B-Language
rules	O
.	O
</s>
<s>
An	O
NRL	O
is	O
merely	O
a	O
list	O
of	O
XML	B-Language
namespaces	I-Language
and	O
a	O
path	O
to	O
a	O
schema	O
that	O
each	O
corresponds	O
to	O
.	O
</s>
<s>
This	O
XML	B-Protocol
format	O
is	O
schema-language	O
agnostic	O
and	O
works	O
for	O
just	O
about	O
any	O
schema	O
language	O
.	O
</s>
<s>
The	O
lowercase	O
form	O
is	O
a	O
generic	O
term	O
and	O
may	O
refer	O
to	O
any	O
type	O
of	O
schema	O
,	O
including	O
DTD	O
,	O
XML	B-Language
Schema	I-Language
(	O
aka	O
XSD	O
)	O
,	O
RELAX	B-Language
NG	I-Language
,	O
or	O
others	O
,	O
and	O
should	O
always	O
be	O
written	O
using	O
lowercase	O
except	O
when	O
appearing	O
at	O
the	O
start	O
of	O
a	O
sentence	O
.	O
</s>
<s>
The	O
form	O
"	O
Schema	O
"	O
(	O
capitalized	O
)	O
in	O
common	O
use	O
in	O
the	O
XML	B-Protocol
community	O
always	O
refers	O
to	O
W3C	O
XML	B-Language
Schema	I-Language
.	O
</s>
<s>
Because	O
attributes	O
cannot	O
contain	O
elements	O
in	O
XML	B-Protocol
,	O
this	O
question	O
only	O
arises	O
for	O
components	O
that	O
have	O
no	O
further	O
sub-structure	O
that	O
XML	B-Protocol
needs	O
to	O
be	O
aware	O
of	O
(	O
attributes	O
do	O
support	O
multiple	O
tokens	O
,	O
such	O
as	O
multiple	O
IDREF	O
values	O
,	O
which	O
can	O
be	O
considered	O
a	O
slight	O
exception	O
)	O
.	O
</s>
<s>
Text	O
content	O
Some	O
XML	B-Language
schemas	I-Language
,	O
particularly	O
ones	O
that	O
represent	O
various	O
kinds	O
of	O
documents	O
,	O
ensure	O
that	O
all	O
"	O
text	O
content	O
"	O
(	O
roughly	O
,	O
any	O
part	O
that	O
one	O
would	O
speak	O
if	O
reading	O
the	O
document	O
aloud	O
)	O
occurs	O
as	O
text	O
,	O
and	O
never	O
in	O
attributes	O
.	O
</s>
<s>
However	O
,	O
there	O
are	O
many	O
edge	O
cases	O
where	O
this	O
does	O
not	O
hold	O
:	O
First	O
,	O
there	O
are	O
XML	B-Protocol
documents	I-Protocol
which	O
do	O
not	O
involve	O
"	O
natural	O
language	O
"	O
at	O
all	O
,	O
or	O
only	O
minimally	O
,	O
such	O
as	O
for	O
telemetry	O
,	O
creation	O
of	O
vector	O
graphics	O
or	O
mathematical	O
formulae	O
,	O
and	O
so	O
on	O
.	O
</s>
<s>
Schema	O
reuse	O
A	O
new	O
XML	B-Language
schema	I-Language
can	O
be	O
developed	O
from	O
scratch	O
,	O
or	O
can	O
reuse	O
some	O
fragments	O
of	O
other	O
XML	B-Language
schemas	I-Language
.	O
</s>
<s>
Various	O
parts	O
of	O
the	O
extensive	O
and	O
sophisticated	O
Text	B-Language
Encoding	I-Language
Initiative	I-Language
schemas	O
are	O
also	O
re-used	O
in	O
an	O
extraordinary	O
variety	O
of	O
other	O
schemas	O
.	O
</s>
