What can you put in the extra slot next to the armor bar in Minecraft

SpigotMC - High Performance Minecraft Mauwh The Inventory javadocs are pretty clear about what you can do here: upload_2022-7-9_

All slot index ranges are inclusive and reflect the indices observed in the Minecraft protocol hotbar It is possible to

minecraft slot A slot is a location in the data of a block entity or entity that an item stack can be placed in

minecraft slot machine The slot system consists of 35 Numerical slots and 6 Custom png with the name of the png file NOTE: The map image must be NO