Ray pretty much nailed it. The only thing I have heard differently is that once you convert a pistol to a rifle it is always a rifle and if you have a barrel shorter than 16 inches overall it is now an SBR.

But an unfinished lower is neither a rifle or a pistol. I wouldn't own an upper than is shorter than rifle length without a pistol lower. That's a recipe for disaster.

When you purchase a lower from an FFL you can have them mark it as a pistol if you choose.

I wished I had known that when I bought my lower. I paid way too much to have a pistol marked lower. I could have saved about 100 bucks. On the lower.

Heck right now they have bushy lowers for under 50 bucks. If my go to FFL hadn't gone under I would buy a couple more.