"lower" is not always the case. More accurate to describe the limitation as "receiver".
The receiver of different items will vary from model type to model type. Basically...whatever part on the real firearm version contains the "action" (firearm term...google it) is considered "the firearm" and thus controlled for import.
Everything else is just an "accessory" of the firearm. The barrel, grips, sights, RIS/RAS, dustcovers, selectors, etc....
M4 - Lower Receiver
SCAR - Upper Receiver