All widgets are contained in a shell widget. This is the top level widget usually. The job of a shell widget is to interface to the window manager.
The application shell is the normal top level for an application and is created by XtVaAppInitialze() and related functions.
There are two other shells: the Override shell which is used for pop-up menus that must be at the top-level. These are not often used by Motif programs.
The Transient shell which is used for dialogs. However Motif repackages this shell so that dialogs become a subclass widget.