Together with the MUI's notification mechanism, this method connects your user interface and your program. If you e.g. want to be informed if the user presses a ''Play'' button, you would have define an id for this action and set up a notification event with MUIM_Notify.
You can use any long word as return id, except from -255 up to 0. These values are reserved for MUI's internal use and for special return values like MUIV_Application_ReturnID_Quit.
Note that MUI will put all incoming return ids onto a private fifo stack and feed this stack to its input methods result code later.