The Stock Keeping Unit, or SKU is a unique identifier for a product, service, or bundle from the perspective of a particular supplier, i.e. SKUs are mostly assigned and serialized at the merchant level.
Examples of SKUs are the ordering or parts numbers used by a particular Web shop or catalog.
Consequently, the domain of hasStockKeepingUnit is the union of the classes Offering and Product Or Service.
If attached to an Offering, the SKU will usually reflect a merchant-specific identifier, i.e. one valid only for that particular retailer or shop.
If attached to a Product Or Service Model, the SKU should reflect the identifier / part number used by the official manufacturer of that part.
Important: Be careful when assuming two Products or Services instances or Offering instances to be identical based on the SKU. Since SKUs are unique only for the same Business Entity, this can be assumed only when you are sure that the two SKU values refer to the same Business Entity. Such can be done by taking into account the provenance of the data. As long as instances of Offering are concerned, you can also check that the offerings are being offered by the same Business Entity.
Usually, the properties hasEAN_UCC-13 and hasGTIN-14 are much more reliable identifiers, because they are globally unique.
See also http://en.wikipedia.org/wiki/Stock_Keeping_Unit.