The difference, as explained to a CQB trained Navy friend of mine (who was cooincidentally on the Cole when it got hit), is this : Sub Tactical is the details of what you do as a person. How you take cover, how broad of a target you present yourself, how much you can see, your ammo situation, etc.
Tactical is what you do that effects your squad, and what they do that effects you.
Strategic is when squads effect one another.
Advanced Wars, by its nature, is a strategic game.