For instance: my XCR-L left Robinson Arm. As a complete RIFLE. Transferred as a rifle to the previous owner and then transferred to me as a rifle.
If if I remove the lower and sell it separately it will transfer as a LOWER RECEIVER but is a previous RIFLE serial numbered and transferred lower. Not a virgin lower.
Hence, buying a stripped or unassembled lower from Robinson Arm. Would transfer as a LOWER RECEIVER which could be assembled as PISTOL or RIFLE and then disassembled and resold as a LOWER RECEIVER again.
Clear as mud?