The NIS+ client-server arrangement is similar to those of NIS and DNS in that each domain is supported by a set of servers. The main server is called the master server, and the backup servers are called replicas. Both master and replica servers run NIS+ server software and both maintain copies of NIS+ tables.
However, NIS+ uses an update model that is completely different from the one used by NIS. At the time NIS was developed, it was assumed that most of the information NIS would store would be static. NIS updates are handled manually, and its maps have to be remade and fully propagated every time any information in the map changes.
NIS+, however, accepts incremental updates to the replicas. Changes must still be made to the master database on the master server, but once made, they are automatically propagated to the replica servers. You don't have to "make" any maps or wait hours for propagation. Propagation now takes only a matter of minutes.