I think "confusing" in this context doesn't mean that the concepts are hard to enumerate or understand, it means they're hard to apply with low error rate in a practical setting.

I don't have a problem with X selection+paste if I'm just using Linux. But using it when connected to a remote machine via various remoting technologies (NX, Chrome Remote Desktop), and then mix in that the host machine is a Mac with its terrible command/control split, and the result is pretty confusing.