Server lists also specify what a client does if it cannot find any preferred servers. A preferred server is any server with a preference of zero, or any server that you have assigned a preference number with nisprefadm.
By default, if a client fails to reach a preferred server, it will then seek out any server it can find anywhere on the network using the search mode described in "Default Client Search Behavior". You can change this default behavior with the nisprefadm -o option to specify that a client can only use preferred servers and if no servers are available it cannot go to non-preferred servers. See "Specifying Preferred-Only Servers" for details.
This option is ignored when the machine's domain is not served by any preferred servers.