Ownership status
Whether the gallery owns a work outright, holds it on consignment, or no longer holds it — distinct from whether it's for sale.
Two questions get conflated in casual inventory: "can we sell it?" (legal right) and "is it available?" (sales process). Ownership status answers the first — owned, consigned in, ex-inventory, third-party — while availability answers the second — available, reserved, under offer, sold, on loan.
Systems that flatten both into one status list force impossible answers ("is a reserved consigned work 'consigned' or 'reserved'?"). Keeping the two axes separate is a small data-model decision that removes a whole category of bookkeeping errors.
In the product: Inventory. Related: Consignment