When product parts are listed as a specified number of copies of a single item, usually identified by a <ProductIdentifier>, <NumberOfCopies> must be used to specify the quantity, even if the number is ‘1’. It must be used when a multiple-item product or pack contains (a) a quantity of a single item; or (b) one of each of several different items (as in a multi-volume set); or (c) one or more of each of several different items (as in a dumpbin carrying copies of two different books, or a classroom pack containing a teacher’s text and twenty student texts). Consequently the element is mandatory, and non-repeating, in an occurrence of the <ProductPart> composite if <NumberOfItemsOfThisForm> is not present. It is normally accompanied by a <ProductIdentifier>; but in exceptional circumstances, if the sender’s system is unable to provide an identifier at this level, it may be sent with product form coding and without an ID.