A Stock Keeping Unit (SKU) is an identification code used by a supplier to uniquely identify a billable item that could be either a product or a service. While they are used for physical products they can also be assigned to intangible items such as repair time or warranties.
To maintain the unique properties of the SKU a single code is often allocated to each and every variant of a product. One product can have many SKUs each identifying a particular variant right down to the last detail.
Using this highly specific code will ensure the product ordered is the one picked from the shelf when the order is fulfilled.