Minecraft wiki entry for Player.dat_format, associated text: All items can be given Modifiers which affect various Attributes of the player or mob wearing or holding them. Note that if an item has default attribute modifiers, these cease to exist if this tag is added (e.g. a diamond sword given an empty AttributeModifiers list no longer provides a boost to damage). Also note that default attribute modifiers do not use this tag, and as such, it does not appear on a natural item.