The unit mastery system gives out rewards for using the same MS for a long time.
Every battle, all units on your deck receive experience points.
Rewards increase in proportion to mastery level.
Any MS can receive mastery experience points, including rental ones.
Pilot's mastery levels can be seen under their respective (機体練度) tab in the pilot card.
Every time you take your deck into battle, every MS in your deck receives 100 mastery EXP per copy.
Using multiple copies of one MS in a deck means it gets 200, 300 or 400 EXP respectively.
Certain MS (like ace custom units) have fixed colour palettes. In their case, you receive GP instead of a mastery colour.
Using multiple copies of the same MS allows you to get to level 10 in 72/48/36 battles respectively. For level 20 it's 222/148/111 battles.