Trust Coins are used to exchange for various rewards in the Item Exchange located under the Shop menu.

Only units with a Trust Master value of 100% can be traded for Trust Coins.
Free units, raid summon units, and EX point units are not eligible to trade for Trust Coins.

The Trust Coin value of a unit is as follows:
5★ base unit = 500 coins
4★ base unit = 200 coins
3★ base unit = 100 coins