The GS1 Identification Key used to identify trade items. The key is comprised of a GS1 or U.P.C. Company Prefix followed by an Item Reference Number and a Check Digit.’ Global Trade Item Number (GTIN) is one of the main building blocks of the GS1 System and is used to uniquely identify trade items, which are products and services that are priced, ordered or invoiced at any point in the supply chain