A centre stop is a small bracket fixed to the driveway under the meeting line of a double swing gate. It has a base plate and an upstand that the bottom of each leaf bears on, so neither leaf can swing past closed. It is fitted after the gates are hung, so it lands exactly where the leaves meet.
The upstand has to stand higher than the gap under the gate, or the leaves pass over it. Automated gates need a physical stop at the closed position and at each open position, so a centre stop is usually paired with an open stop or hold-back catch for each leaf.
CAD60 places the centre stop on the side the leaves close toward and sets out the open stops on the Swing Plan page, measured from each leaf's hinge pin.