That meant to say "shoulder a pistol brace". It's a recent ruling from April 2017. https://www.sigsauer.com/press-releases/atf-clarifies-ruling-pistol-stabilizing-braces/
Pretty much every pistol brace manufacturer announced it. If you actually contact the ATF about it, their rules are that the pistol brace can't be longer than 13.5" and it can't be permanently attached to the buffer tube. Other than that, if you shoulder a pistol brace, it does not constitute a redesign of the firearm, it is still classified as a pistol, and is not an NFA firearm.