A SKU barcode is a unique identifier for a product or service available through a supplier. It is located on the product itself or on its packaging. Vendors list SKUs in their databases whether or not they are on the product in question.
Vendors use SKUs, or Stock Keeping Units, for data management purposes and inventory tracking. SKUs are always associated with billable items, and therefore may apply to subunits of a large shipment if the subunit is not billable on its own. For example, if a box of twenty paper towel rolls is a billable entity, the box has a SKU barcode but the individual rolls do not.