Minecraft wiki entry for Player.dat_format, associated text:  AttributeModifiers: Contains Attribute Modifiers on this item which modify Attributes of the wearer or holder (if the item is not in the hand or armor slots, it has no effect).
: A single Attribute Modifier.
 AttributeName: The name of the Attribute this Modifier is to act upon.
 Name: The name of the Modifier. Has no known effect.
 Slot: Slot the item must be in for the modifier to take effect. "mainhand", "offhand", "feet", "legs", "chest", or "head".
 Operation: Modifier Operation. See Attribute Modifiers for info.
 Amount: Amount of change from the modifier.
 UUID: The modifier's UUID, stored as four ints. Used when equipping and unequipping the item to identify which modifier to add or remove from the entity.