Reversivity Coins are a special type of currency used in trading with Operator when buying Treasure Bags from her.
Obtained from bosses, coins are ranged in tiers from 1 to 6, with 6 being the highest. The further the boss stands in progression, the higher its coin tier and amount of coins dropped would be. You would get notified in chat when the Reversivity Coin gets added to your storage.
Coins are not an item in the normal way; they would only appear when you go trading with Operator, inside your Defender's Forge. When you're done with trading, they would get stored back inside the "internal" storage.
You can view your current amount of coins while the Operator's Shop Changer is open.
In there, you can also trade higher tiers for lower ones at the rate of 1 → 2.
For example, 1 T6 Coin is traded for 2 T5 Coins. This doesn't work the other way around.