My mind is blown How do I add an inventory slot?

I have been playing and recently noticed this in in my inventory upload_2019-6-7_ I have no idea what that slot is, can any one

Inventory items are stored per-slot, with customisable metadata to support item uniqueness Overrides default weapon-system with weapons as items Weapon

inventory slot png Slot?, one slot, minecraft one slot, one inventory slot, beating minecraft with one slot, 1

door slot Checking for an item in certain Inventory slot #1 Jan 17, 2014 imagesbb2Items_slot_ Any help?