You have to list caliber, but I don't know if that means you can change said caliber once approved. Just don't know if it is ever an option or notDoesn't matter it seems; just barrel lengths, whether it has or doesn't have a stock, and does it fall into any of the NFA categories (since the calibers are for rifled barrels, that leaves SBRs and AOWs)
Edit. I don't know if the ATF would accept "multi cal" on a SBR lower markings for "making" but... that's more @wired Wheelhouse