Clients have player indexes for each player on their machine.
Server also has player indexes for each player on its machine, but because it is the server, I think that means that if you have 5 machines with 3 players on each machine, that would mean the server has to keep track of 15 players. I don’t know yet if this is how it works, but I am trying to create a game where this is possible.
I also hope that the online subsystems allow cross-platform sessions. I read somewhere that Session types are hardware-specific, so this might no be possible, which would make me sad.