Complex sentence: The namespace roots do not have to reside on domain controllers, they can reside on member servers, if domain controllers are not used as the namespace root servers, then multiple member servers should be used to provide full fault tolerance.
Split into simpler sentences: The namespace roots do not have to reside on domain controllers; they can reside on member servers. If domain controllers are not used as the namespace root servers, multiple member servers should be used to provide full fault tolerance.