AppMenu (appmenu.el) is a framework for creating cooperative context
sensitive popup menus with commands from different major and minor
modes.

By default the popup menu is on [apps] and [mouse-3].

The entries from ##[menu-bar]## for the major mode and activated
minor modes are added to the popup menu, but this can be
customized.

You can add whatever context sensitive menus you want. 
This may come in handy if you are writing your own mode.
By using AppMenu your mode's use of ##[apps]## will not collide with other modes' use of it (if they use it too of course).

It is part of [NxhtmlMode nXhtml] where it used.