The lower is the firearm. If you purchase a lower, it is registered either as a stripped lower, or a complete. If you complete a stripped, you have to notify the Centre. When registered as a complete firearm, CFC wants to know barrel length. Some people have multiple barrels of different lengths, so that record is updated to either have all the lengths the person has, or "various", if they'll allow it. That'll allow the lower to be attached to whatever upper you want to use that day.