Minecraft wiki entry for item, associated text: Slot Slot Numbers Restrictions
armor.chest armor stands, mobs, and players only (though not all mobs will show or make use of the items)
armor.feet
armor.head
armor.legs
weapon.mainhand
weapon.offhand
container.<slot_number> 0–53 players, item frames (slot 0), and minecarts only
enderchest.<slot_number>
0–26 players only
hotbar.<slot_number>
0–8
inventory.<slot_number>
0–26
horse.saddle
horses, donkeys, and mules; <item> must be a saddle or air to remove
horse.chest donkeys/mules and llamas; <item> must be a chest or air to remove
horse.armor
horses; <item> must be a type of horse armor or air to remove
llamas; <item> must be a type of carpet or air to remove
horse.<slot_number>
0–2 llamas with chests
0–14 donkeys/mules with chests
villager.<slot_number>
0–7 villagers and piglins only