XmCascadeButton
The entry in a Motif menu bar that drops down a menu — or, inside a menu, opens a submenu marked with a small arrow.
A Motif menu bar is an XmRowColumn of the menu-bar type, and each heading in it — File, Edit, Help — is an XmCascadeButton. The button itself does nothing. Its XmNsubMenuId resource points at a pulldown created with XmCreatePulldownMenu, and that is what appears when the button is activated. Put the button inside another menu and Motif draws an arrow on the right; the cascade becomes a submenu.
XmNcascadingCallback runs just before the menu is shown, which let programs fill in or grey out entries at the last moment instead of keeping the menu in sync all the time. The Help menu was pushed to the far right of the bar via the menu bar's XmNmenuHelpWidget resource, a convention straight out of the Motif Style Guide.