Military Med Cats
If memory serves it is Air, Ground, Zone so A1 fit to fly as passenger or aircrew in any kind of aircraft, G1 Fit for any type of ground role, Z1 fit to serve anywhere in the world.
I am sure there are more specific details than this but that is the basis of the cat. For example I am pretty sure that the highest category of A that can be held by ground personnel is A4.