The userID is expected to be unique and It is used by callstats.io to distinguish between different endpoints. You can decide if the user ID is ephemeral or an actual username.
In a peer to peer communication between Alice and Bob: whenever reporting stats from Alice's point of view, please specify Alice as localUserID, and Bob as remoteUserID. The opposite is needed for Bob's point of view.
More information about userID can be found here.