One point. Good for short, light carbines, transitioning quickly to the support side shoulder. Attached, preferably, at the castle nut location give or take. Downside, not so good for admin/slung maneuvering.
Two point. Good for slung/admin stuff as it stabilizes the gun better. Attached, typically, at the barrel nut at the front and either the castle nut or the rear of the stock. Downside, slower transitions to support side shoulder.