Subordinate reference replicas are system-generated replicas that don't contain all the object data of a master or a read/write replica. Subordinate reference replicas, therefore, don't provide fault tolerance. They are internal pointers that are generated to contain enough information for eDirectory to resolve object names across partition boundaries.
You can't delete a subordinate reference replica; eDirectory deletes it automatically when it is not needed. Subordinate reference replicas are created only on servers that hold a replica of a parent partition but no replicas of its child partitions.
If a replica of the child partition is copied to a server holding the replica of the parent, the subordinate reference replica is automatically deleted.