Messaging Server 5.x and Calendar Server 5.x installations use Schema
The Directory Information Tree (DIT) organizes LDAP entries in a tree
structure with nodes representing domains, subdomains, users, groups, and resources.
Schema 1 generally uses a two-tree structure:
The Domain Component (DC) Tree contains domain nodes decorated with all
the pertinent domain attributes.
The Organization (OSI) Tree contains organization nodes that have the
user, group, and resource entries underneath them.
Messaging and Calendar servers look up entries by accessing domain information
in the DC Tree and using that information to find the appropriate entries in the Organization