Domains can be controlled by the Server Administrators and by the Domain Administrators - Accounts in the same Domain that are granted some Domain Administrator Access Rights. You may choose to grant administration rights for this Domain to Domain Administrators created in a different Domain. In this case the name of that other Domain should be entered into the Administrator Domain Name field:
If this field is not empty, the Domain Administrator Accounts created in this Domain and the Domain Administrator Accounts created in the specified Domain can be used to administer this Domain.
See the System Administrator section for more details.