The version of the wire determines if two agents of the system (driver, servers, and mongos) can interact. For each agent, there is a version that they are at (max) and there is a minimum version that they are compatible with. If two agents have on overlap of a version, they can talk. In practice, this may prevent a too old server to join a cluster or a too old application to talk to a server.
Each agent has a fixed range that never changes.
For a mongod and a mongos, the version will be reported on the 'isMaster' command:
'isMaster.maxWireVersion' reports the version the mongod/s are at
'isMaster.minWireVersion' reports the oldest version that the mongod/s can understand
For now, versions are only going to be reported on 'isMaster'.