SKU
Also called: stock keeping unit
A SKU (stock keeping unit) is the unique code that identifies one exact sellable variant of a product. Not the product, the variant: an 11oz white mug and a 15oz white mug are two SKUs, and so are the same shirt in medium and large.
In print on demand the SKU is what actually gets fulfilled. When an order arrives, the platform matches the buyer's chosen options to a single SKU, and that SKU determines the blank pulled from the shelf, the print method, and the cost.
This matters most when you connect a store. If your marketplace listing has three sizes and four colours, that is twelve variants, and each has to map to a real SKU on the fulfilment side. Unmapped variants are the most common cause of an order that cannot be produced automatically.