I'm under the impression that the trapping instructions are for when the ball is further away from your goal, Your opponent's build up phase as it were.
So the crossing instructions are for your defensive 1/3 and the trapping instructions are for the middle and your opponent's defensive 1/3.
I could be wrong.
e.g You might want to force the other team wide and them win the ball in wide areas so you would have the instructions: Trap outside - Stop Crosses