NetworkRunner.StartGame() - what exactly do CustomCallbackInterfaces do and how do I use them?

Options

There is alot of interesting things in StartGameArgs, but many of them are impossible to use without proper documentation. A documentation that I find sorely lacking.

I went through some of the demos and not even here is a thing like CustomCallbackInterfaces used, so there are no examples at all.

I would like to know what I can do with the customcallbackinterfaces. Can someone explain this structure in detail?