There was a pleasant surprise waiting for players when Call of Duty: Black Ops 6 went live. XP tokens from previous Call of Duty games were there sat in everyone's inventory, making it easier to face the grind and level up characters or weapons. Usually, players can get these by completing limited time events, finishing the battle pass, or sometimes via collaborations.
Legacy XP tokens aren't unheard of. Call of Duty: Modern Warfare 2 and Modern Warfare 3 both let players use previously purchased or unlocked items and cosmetics as well as double XP tokens from various Call of Duty games.
