If you are running applications (such as provisioning scripts or tools) developed at your site that rely on Schema 1, and it is not a trivial task to convert the applications to use Schema 2, you can choose to migrate to Schema 2, compatibility mode, as a first step before you migrate to Schema 2, native mode.
Schema 2, compatibility mode, retains the two-tree structure of Schema 1.
The Messaging and Calendar servers, and your own user-developed applications, continue to access the LDAP directory exactly as they did in Schema 1:
They use the DC Tree to access the user and group nodes in the Organization Tree.
From the perspective of the Messaging and Calendar servers and user-developed applications, Schema 1 is still in place.
At the same time, Schema 2, compatibility mode, enables you to use the Delegated Administrator console and utility (commadmin) and Access Manager features such as single sign-on (SSO). During the migration to Schema 2, compatibility mode, Access Manager object classes, attributes, and ACIs are added to the appropriate nodes in the Organization Tree.