Pilot Archive

The clearest way to state it, is that you get a mastery bonus for each pilot profession you master. If your first Ace is freelance, you will get x2 for your next squad, even if it's another freelance. However, since that's the same Pilot profession, you won't earn another Mastery bonus for completing it. When you master Imperial or Alliance Pilot, you get a second and then third mastery bonus.

See link in sig