Minecraft wiki entry for Scoreboard, associated text: The AllowFriendlyFire option controls whether or not members of a team are able to damage each other. This defaults to true, leaving PvP mechanics unchanged—ergo, players can harm their teammates. When set to false, however, players on the same team are prevented from directly damaging each other with melee attacks, bows, and Splash Potions of Harming. Note that players on the same team may still inflict negative status effects on each other with potions, even if <AllowFriendlyFire> is false. Some non-player entities in a team are also affected by this.