Works identically to the system menu.
If you use it in things like select/selgosub, then you'll end up with something not very different from the title screen.
(In this case, it would probably be better to temporarily toggle off right-click menu functionality through rmode)
The available commands are exactly the same as the ones utilize by rmenu.
If you call "systemcall rmenu", this is the same as calling the right-click menu itself.
You'd use this when you wanted right-click menu functionality during a button wait state. Once you exit this right-click menu, the interpreter continues its execution one line below the systemcall command.
(This can leave the screen and the interpreter stack in an inconsistent state, so be careful when using this command.)