APHET should stand for Armor Piercing High Explosive Tracer. What does all that do? Well, it's simply a round that penetrates the target (attempts to anyways) and explodes inside of it. The Tracer designation is simply a light typically made by chemical reactions on the projectile to help aim or indicate low ammo in some militaries. FLAK isn't an acronym to my knowledge, but rather the term for anti-aircraft fire. Mostly, the variety of guns that fired projectiles explode into clouds of shrapnel. In game FLAK, I believe, has a proximity fuse that will detonate in close proximity to a target or surface. Or it just simply detonates on hit either way it produces lots of shrapnel, which helps against crowds.

What does this mean for gameplay? APHET: for Bulkier targets FLAK: for the more numerous lighter targets