Minecraft wiki entry for Target_selectors, associated text: Slot Position Acceptable Slot Numbers Restrictions
slot.weapon.mainhand 0 mobs (including armor stands), and players only (though not all mobs will show or make use of these slots)
slot.weapon.offhand 0
slot.armor.head 0
slot.armor.chest 0
slot.armor.legs 0
slot.armor.feet 0
slot.hotbar 0-8 players only
slot.inventory 0-the number of slots in inventory entities with minecraft:inventory component only
slot.enderchest 0–26 players only
slot.saddle 0 horses, donkeys, and mules that are tamed only
slot.armor 0 horses only
slot.chest 0–the number of slots in chest donkeys and mules with chests only
slot.equippable 0-the number of equippable slots entities with minecraft:equippable component only[check the code]