Microsoft Knowledge Base Article 257218 contains the following summary:

The Windows 2000 schema contains a large number of object attributes that administrators can choose for use. The attributes normally required by Active Directory are enabled by default when the first domain controller is installed, and have the Index this attribute in the Active Directory check box selected in their properties.

You can use the Active Directory Schema snap-in in Microsoft Management Console (MMC) to select the number of attributes and the specific attributes you want. In most cases, however, there is no need to modify any of these attributes. Carefully consider any changes to these default settings before making the changes.

NOTE: You should make changes or additions to the schema only after careful consideration and planning. Once added, they cannot be deleted, only disabled. Adding additional attributes increases the time required for their replication. Carefully consider the net results of any changes.

For additional information about the schema, click the article number below to view the article in the Microsoft Knowledge Base:

216060 Registry Modification Required to Allow Writing to Schema