The two leaves of a double swing gate meet with a gap between them, typically 15 to 25 mm. A stop strip is a flat, often 40 or 50 mm wide and 5 mm thick, welded down one meeting stile so it covers that gap and laps onto the other leaf. It stops the leaves swinging past each other and closes the gap to the eye.
It is normally welded to the passive leaf, on the face the active leaf closes against. On a gate that opens in, that is the outside face. The strip has to be wide enough to cover the gap, lap the other stile and still leave room to weld.
CAD60 draws the stop strip as a cut part, works out its face from the way the leaves open, and sets out its width and lap on the Meeting Stile Detail page.