What each one does
Crumb decides what the product looks like, how it bites and how much oil it takes. Pre dust decides whether any of that stays attached. They are judged together and they do entirely different jobs.
What decides it
Where the separation is. Coating that lifts in sheets, taking the batter with it, is an interface failure and belongs to the pre dust. Coating that crumbles away while the batter stays is a crumb or a frying problem.
Where the obvious answer is wrong
The obvious answer is a stickier batter. It usually is not. A thicker batter on a wet surface adheres no better and makes the coating heavier, and the extra weight pulls it off faster during freezing.
