ATF regulation.
Actually it's federal law, not just ATF regulation.
Title 18 Part I Chapter 44 Section 921 (a) (6):
(6) The term “short-barreled shotgun” means a shotgun having one or more barrels less than eighteen inches in length and any weapon made from a shotgun (whether by alteration, modification or otherwise) if such a weapon as modified has an overall length of less than twenty-six inches.
Along with relevant parts of 18 Part I Chapter 44 Section 922 which actually make various things illegal with respect to short barreled shotguns.