Such servers may be hidden, starting up, or recovering. They cannot be queried, but their hosts lists are useful for discovering the current replica set configuration.
Servers may be identified as such in at least three situations: briefly during server startup; in an uninitialized replica set; or when the server is shunned (i.e. removed from the replica set config). They cannot be queried, nor can their host list be used to discover the current replica set configuration; however, the client may monitor this server in hope that it transitions to a more useful state.