The userID is used by callstats.io to distinguish between different endpoints. You can choose whether the userID should be ephemeral or an actual username. However, it is expected to be unique.
In a peer-to-peer call between two individuals Alice and Bob, whenever reporting statistics from Alice, specify Alice as localUserID and Bob as remoteUserID. Similarly, when reporting statistics from Bob, specify Bob as localUserID and Alice as remoteUserID.
You can read more about userID in our API documentation.