There was a big discussion when this was dropped. The consensus was that when a jet was going to land on a flat top the flight controller would tell him to ‘call the ball’. IIRC, there are red and green balls (for port and starboard) the pilot sees to line up for the landing, he radios the controller, ball acquired and tells which color. Hope that makes sense.
Now it’s coming back from the dim dark recesses of my memory...thank you.