50D C.Fn 3-2 Lens AF stop button?

So where is this mystical AF stop button located? Anyone else have any experience with this CFn? Setting number 4 "ONESHOT <> AI SERVO" caught my eye. Or is this tied into the * button focus?

I tried pressing the star button by itself and in combination with the shutter button and it did nothing. I hate the whole back button focus separate from exposure thing. I interpreted this as second button since the * button is clearly labeled AF-On, and this CFn clearly identifies an "AF-Stop button"

The AF stop button is found on Super Telephoto lenses. The four buttons that you see near the front element on lenses such as the 400L are the AF stop buttons.

This is a button fitted to super telephoto lenses only (as the note under C.Fn III-2 in the manual says)

I see, thanks! My manual is in storage across town hehe