Ammunition is a new kind of auxiliary item that can be used in Boss fights.
Types of ammunition:
Medicine — partly restores a character's health.
Magmium — a grenade that deals damage to Bosses. The damage dealt depends on your Void Power.
Destabilizer — a grenade that stuns Bosses for a while.
Ammunition can be obtained in various ways — get it from rewards or buy it in the Store. Only one type of ammunition can be used in each round. You can change this between battles and via the Raid map.