This documentation is outdated.
Passthrough messages are an easy way for Clients to communicate with each other. It's a WIP feature and currently only supports single Client passthrough.
If a Client calls Send or SendToLocalClient (and their target variants) (the only two overloads that support only one target client), the message will be sent to the server who will then pass it through to the target Client. Every messageType that you want to support this passthrough feature has to be specifically marked as a pass-through channel. (It means they behave like normal messageTypes but they also allow passthrough). This means that you can easily have logic in your game code where you fully trust the data you received because it was received on a non-passthrough messageType. But also have fast communication between Clients.
The first argument of the messageListener is the verified by server sender of the pass-through message.