A Stock-Keeping Unit or SKU, is a number or code used to identify each unique product or item for sale. It is a unique identifier for each distinct product and service that can be purchased. Each SKU is attached to an item, variant, product line, bundle, service, fee, or attachment. SKUs are often used to refer to different versions of the same product. For example, a store carrying the game Guitar Hero 3 might have six SKUs, two for each of the three platforms—one with and one without a guitar controller.