Messaging Server 5.x and Calendar Server 5.x installations use Schema 1.
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 Tree.