Wikipedia has much better information about guns than most of the people talking about them in politics, generally speaking.
It's not too surprising, considering the way the rules are written at the ATF. There's basically zero logical thought that goes into pistol vs rifle vs felony:
ATF rulemaking can be unintuitive and arbitrary but there really is a level below it occupied by people who have dedicated a significant chunk of their lives to trying to restrict firearm ownership, who genuinely seem to believe that Die Hard, Rambo, and Spaghetti Westerns are real life. Politicians who can't answer basic questions about their legislation, who have to be told live on air that magazines can be repacked, that just make up impossible crime statistics. Yeah it's stupid that the ATF has decided that vertical grips are a rifle feature but angled grips aren't, but it gets worse.
There's no legal definition per Congress. Generally speaking, braces are intended to stabilize a pistol against your arm [0], whereas a rifle stock is meant to stabilize against your shoulder. However, braces can technically be "misused" such that the rear of the brace fits against the shoulder, meaning it is used as a stock. Likewise, the distinction is so small something as simple as a sling attachment to the stock could make it a brace, or an articulation that could be used as a cheek rest turn a brace into a stock, converting a pistol into a rifle or vice versa.
For awhile, the only way to know the difference was for the manufacturer to submit an NFA and hope.
The ATF has been in court (and lost) quite a bit [1] over this.
A "pistol brace" is designed and "intended" to be braced against your forearm to stabilize the "pistol" in a way that allows you to shoot a particularly large and heavy "pistol" with one hand. The ATF said this was fine, although I think they really regret that now.
Stock goes against the shoulder. Brace goes against the elbowpit. If you let the brace touch your shoulder, your braced pistol suddenly becomes an unregistered SBR, and you become a felon. Oopsie!
It's not too surprising, considering the way the rules are written at the ATF. There's basically zero logical thought that goes into pistol vs rifle vs felony:
https://www.reddit.com/r/Firearms/comments/a4gnr3/makes_perf...
(Sorry for the reddit link, it's a common image but that was the first url I found from a quick search that had it up front and center).