Minecraft wiki entry for Item_modifier, associated text: set_attributes—Add attribute modifiers to the item.
 modifiers:
: An attribute modifier to add.
 name: Name of the modifier.
 attribute: The name of the attribute this modifier is to act upon.
 operation: The operation of this attribute modifier. One of addition, multiply_base, or multiply_total.
 amount: Number provider. Specifies the amount of the modifier.
 id: Optional. UUID to assign the modifier, in hyphenated hexadecimal format. If none specified, a new random UUID is generated.
 slot: Sets the slots the item must be in for the modifier to take effect. One of: mainhand, offhand, feet, legs, chest, head.
 slot: To choose randomly from a set of possible slots. List contains strings of one of the above slot choices, to choose randomly from. The random selection cannot[verify] be weighted by listing the same slot multiple times.