The fixed part of the event data goes under different names, depending on which source file, work log, bug report, etc. you are reading:
Sometimes it is called the "fixed data" part, as in this discussion.
Sometimes it is called the "post-headers" part.
To make things notationally interesting, sometimes the
fixed data part is referred to as the "event-specific
headers" part of the event. That is, the word "header" is
used in reference to a portion of the data part. One
manifestation of this notational phenomenon appears in
log_event.h, where you will find the
defined as 19 (the header length for v3 and v4), plus
other symbols with names of the form
XXX_HEADER_LEN for different event
types. The former symbol is the size of the event header
(always 19). The latter symbols define the size of the
fixed portion of the data part that is to be treated as
the event-specific headers. For example,
ROTATE_HEADER_LEN is 8 because a
ROTATE_EVENT has an 8-byte field in the
fixed data part that indicates the position in the next
log file of the first event in that file.
The variable part of event data also goes under different names, such as the event "payload" or "body."