A combination of subassemblies, assemblies, components, modules, and parts connected in such a manner as to be a self-contained unit that, although part of an end item, is capable of operating independently of the end item. Major assemblies are separately identified by type, model and series and assigned item ID numbers (SB 700-20). Examples are receivers or receiver-transmitters in radio sets and machine guns or other weapons in secondary, armament subsystems of combat vehicles