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The queue keeps the game session's Max Players at Total Slots. With Publish Queue To Session on, it also writes the number of waiting players (party members included) into the online session, under Session Queue Length Key (SQ_QUEUE). A server list can then show "64/64 (+12 waiting)".

Read it with Get Session Queue Length on a session search result.

For other server lists, for example Steam server rules or your own master server, bind On Queue Status Changed on the Server Queue Subsystem and publish the numbers yourself.