Let's say you created a new device--a Media Player. For anybody to use the device, there must be a menu, or remote control on the Orbiter that appears when the user tries to turn on your device. If there is an existing menu that is appropriate, say for another media player, you can always re-use it.
The Orbiter will then send you the commands when the buttons on that menu are touched. You may also want to create your own menu, or series of menus, using Designer. This page is where you indicate what are the primary, or topmost menu(s) that can be used to control your device. If only 1 menu (or Design Object) is specified, that will always be used. If you specify more than 1, perhaps there are multiple UI's that will work, then the user will be given the choice of choosing the one he wants to use to control your device from the list you create here. Then, when a button is added to an Orbiter to use this device, touching that option will bring up the menu. Note that you may have a whole series of menus to control your device which reference each other.
In the Media Player example, there may be a "sound settings" menu. You should only list the menus here that are valid entry points. Sound settings would not normally be a valid entry point--it would be reached by clicking a button on another menu that was the entry point'