Again, this topic ties into the Active Directory Implementation and Administration portion of the exam. Active Directory requires DNS to function correctly. DNS support for SRV records is the only absolutely mandatory requirement for Active Directory to function. If your DNS servers do not support this, you would need to set up all required service records for all domain controllers manually, a potentially long and arduous process. Just to reiterate, Windows 2000 uses service records in DNS to locate domain controllers in specific domains, domain controllers in the same site, global catalog servers, key distribution centers, and more.
Class specifies the DNS record class value, almost always IN for Internet.
Priority specifies the priority level of the server. Clients will attempt to contact the server with the lowest priority value. Weight is a load-balancing feature. If multiple servers have the same priority, clients choose SRV records with higher weights. Port specifies the port number that a particular service listens on.