[1] Such identifiers are used to look up product information in a database (often by entering the number through a barcode scanner pointed at an actual product) which may belong to a retailer, manufacturer, collector, researcher, or other entity.
Note: the equivalent UPC small format barcode, UPC-E, encodes a GTIN-12 with a special Company Prefix that allows for "zero suppression" of four zeros in the GTIN-12.
All books and serial publications sold internationally (including those in U.S. stores) have GTIN (GTIN-13) codes.
The book codes are either constructed by prefixing the old 10-digit ISBN with 978, and recalculating the trailing check digit, or from 1 January 2007 issued as thirteen digits starting with 978 (eventually 979 as the 978 ranges are used up).
Each type of trade item is given its own GTIN, with the understanding that there is a potential need to retrieve predefined information from such items; this product or service may be priced, ordered, or invoiced at any point in the supply chain.