Extending the Schema
When the existing class and attribute definitions in the schema do not meet the needs of your organization, the schema can be extended by adding or modifying schema objects. The Active Directory schema can be extended dynamically. That is, an application can extend the schema with new attributes and classes and use the extensions immediately. Schema updates are accomplished by creating or modifying the schema objects that are stored in the directory. This allows you to make the objects that are meaningful to your organization available throughout the enterprise.
As is true for every object in Active Directory, schema objects are protected by access control lists (ACLs), so only authorized users can alter the schema. (For more information about ACLs, see "Access Control" in this book.)
Adding or modifying class or attribute definitions in the schema involves adding or modifying the corresponding classSchema object or attributeSchema object. The operations that are involved in this process are similar to adding or modifying any object in Active Directory, except that additional checks are performed to ensure that changes do not cause inconsistencies or problems in the schema in the future.