Indeed, it’s not clear which of the lists does what. One simple suggestion I can think of is to just use checkboxes.
Or add a little label above the lists: “Available”, “Selected”.
Relying on my knowledge to educate the Virtualmin users about this is not the right solution. The UI should be intuitive and understandable, without resorting to asking someone, or trying to see how the functionality behaves in order to learn it.
