Stock Keeping Unit that serves as a unique identifier for an item. Many merchants assign a SKU number to an item of a specific type, size, and color. This way, they can keep track of how many products of each type, size, and color are selling, and they can re-stock their shelves according to customer demand.
You can include a SKU when you list any item, and then use the SKU instead of (or in addition to) using the ItemID to track your inventory on eBay.
For fixed-price items, if you set Item.InventoryTrackingmethod to SKU, you can use the SKU instead of ItemID as a unique ID when you revise the listing.
Only returned when the listing included a SKU.