The book industry uses the Bookland EAN code because it can render the ISBN into a machine readable symbol (barcode). This causes problems, however, when books are sold in retail outlets other than book stores. Supermarkets, drug stores, department stores and other retailers often sell books, but are not properly equipped to scan the Bookland EAN symbols. A publisher selling to a non-book retailer might be asked to provide books marked with the Universal Product Code (UPC).
The Book Industry Study Group has articulated clear guidelines about how publishers should do this. Only in the case of mass-marketed, rack-sized paperback books should a book have two different barcodes printed on its covers. In that case, the UPC should appear on the back cover (Cover 4) and the Bookland EAN code is printed on the inside front cover (Cover 2). In all other instances, books should be marked with only one barcode, either EAN for the bookstores, or UPC for non-book retailers.
A small number of publishers accomplish this by actually printing two different runs of a book: one printed with an EAN on the back cover, the other with the UPC. This represents too costly a solution for most publications. The proper way to handle the EAN/UPC problem is to print one barcode on the back cover and then label over the printed barcode with its opposite symbol when necessary. For example, a book which is sold mostly in bookstores will have the Bookland EAN code printed on its cover. When that book is sold to non-book retailers, the publisher will need to cover the EAN code with an adhesive label with the UPC printed on it. Printing two different barcodes on the same cover of any book invites confusion among those who scan the barcodes and should be avoided in every instance.