Version 2 and Version 3 Negotiation

Because NFS servers might be supporting clients that are not using the NFS version 3 software, part of the initiation procedure includes negotiation of the protocol level. If both the client and the server can support version 3, that version will be used. If either the client or the server can only support version 2, that version will be selected.

You can override the values determined by the negotiation by using the -vers option to the mount command (see the mount_nfs(1M) man page). Under most circumstances, you should not have to specify the version level, as the best one is selected by default.