Save the World

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A Teammate XP Boost is used to give a teammate Commander XP boost points. The player will earn an additional 30% more experience during a mission and the other players, including the player who used the boost, will gain an additional 5%. If more than one player uses a Teammate XP Boost, the effects will stack. 

Battle Royale

In Fortnite Battle Royale A Teammate XP Boost. It is used to boost a duo or squad member XP during a match to progress their Season Level. Teammate XP Boosts are acquired at certain tiers in the Battle Pass. You will receive these XP boosts when in a Duo or Squad with someone who has one.