Menustrip objects defined for the application are used as menu for every window of the application, as long as the window doesn't define its private menu.
MUIA_Application_Menustrip replaces the old and obsolete MUIA_Application_Menu tag.
Usually, you will create the menu object with MUI's builtin object library from a gadtools NewMenu structure, but its also OK to define the menu tree ''by hand'' using the Family class.