Its one of those things you kind of learn over the experience of time you play the game. There are certain aspects that are good for score and bad for for it.
Fighter- is all about how many kills you can get per sortie, as fast as you can, with the best aim possible, while landing that sortie to get more fighter points and better K/D. In other words, get a lot kills quickly with good aim and don't die
stay high, fast, and don't get caught in the "gang"
Attack- is more about how much destruction you can do to a base, town, strat, CV group, by placing your bombs correctly to insure good damage and hit percentages per sortie. You also need to get air to air kills with basically the same motives as fighter rank. It is basically being good at dive bombing to get the most damage while gettimg a few fighter skills all in one sortie. Landing is always the key.
Bombing- how accurate your bombs are, how much damage you do per sortie, and capturing bases. Kills don't really matter. You just need to focus on getting as much damage as you can per sortie while landing it.
Vehicle- i hate vehicles I let others explain.
Everything is per sortie and landing is crucial to success.