Minecraft wiki entry for replaceitem, associated text: Slot Slot Numbers Restrictions
slot.weapon.mainhand <slotId: int> any number mobs (including armor stands), and players only (though not all mobs will show or make use of the items)
slot.weapon.offhand <slotId: int> any number
slot.armor.head <slotId: int> any number
slot.armor.chest <slotId: int> any number
slot.armor.legs <slotId: int> any number
slot.armor.feet <slotId: int> any number
slot.hotbar <slotId: int> 0–8 players only
slot.inventory <slotId: int> 0–the number of slots in inventory entities with minecraft:inventory component only
slot.enderchest <slotId: int> 0–26 players only
slot.saddle <slotId: int> any number horses, donkeys, and mules only; itemName: Item must be a saddle
slot.armor <slotId: int> any number horses only; itemName: Item must be a type of horse armor
slot.chest <slotId: int> 0–the number of slots in chest donkeys and mules with chests only
slot.equippable <slotId: int> - unsupported