A schema is the set of server rules that define the structures, contents, and constraints of a directory information tree (DIT).
The schema guarantees that any new data entries or modifications meet and conform to these predetermined set of definitions. It also reduces redundant data definitions and provides a uniform method for clients or applications to access its server objects.
The server includes a default set of read-only schema files that define the core properties for the server. The administrative console provides a Schema Editor to view existing schema definitions and add new custom schema elements to your DIT.
Any attempt to alter a schema element defined in a read-only file or add a new schema element to a read-only file results in an Unwilling to Perform result.