NDS replica synchronization ensures that changes to NDS objects are synchronized among all replicas of the partition. This means that any server that holds a replica of a partition must communicate with the other servers to synchronize a change.
In NetWare 6, NDS replica synchronization is more efficient and produces less network traffic than previous versions, because instead of automatically synchronizing replicas, servers are queried to find out if they are synchronized or not. If a server is out of synchronization, the update is sent. But if the server is synchronized, there is no need to send the update.