What is the Repair Cupboard?
The Repair Cupboard (Tool Cupboard) controls building rights and your base territory.
Whoever accesses it controls your base, so always lock it.
It now has 24 slots for storing upkeep resources - keeping it stocked prevents your structures from decaying.
Only one can be placed per building, on a foundation.
Anyone who opens your cupboard is authorized on it automatically — there's no button to press and no confirmation. This is why locking the cupboard matters: an unlocked one hands building rights to the first player who reaches it.
Only the player who placed the cupboard can take authorization away. Standing at it, they can either remove a single player from the authorized list, or use CLEAR to wipe the whole list at once. They can't remove themselves, and this right can't be passed to anyone else.
Monitor its durability, as it can be damaged.
Its build-zone covers the structural elements of the building it's in. See "How do building privileges and build zones work?"