So after those lobbies have been decided, that is who will race in what room. The final results will then be combined together. So drivers in lobby A will finish 1-15, and drivers in lobby B will be 16-31.
If you still don't get it then i'm not sure how much more I can explain. As already said, if it confuses you, then it shouldn't affect you. All you need to do is drive as fast as possible. Beat as many people as possible, but also try and learn who is in your class and beat them too. If you have to do a TT event, I will let people know, and on the night I will post you races in what lobby. All people need to do is follow instructions and understand that.
EDIT - Also, points system has been updated to involve all drivers