Its pretty easy really.
If the player touches it, and then the ball gets passed around half a dozen other players and back to him, then if the umpire calls it a mark, it is a mark, and the umpiring spokesman will confirm that it was the correct decision.
On the other hand, if a player catches and controls the ball, but the umpire thinks that maybe someone else got a touch in somewhere, and calls, "Play on", then that is not a mark, and the umpiring spokesman will confirm that it was the correct decision.