Minecraft wiki entry for Player.dat_format, associated text: There are two ways enchantments are associated with items; the first way is that the item is actually enchanted and the enchantment affects the behavior of the item, and the second way is that the item is an enchanted book which simply stores the enchantments without actually affecting the behavior of the item. There is also the RepairCost tag which tracks anvil usage for items, making them more costly with every use of the anvil.