What did the book number mean?

The book number usually referred to the book's International Book Number. The first six digits represent the country or region code, and the last seven digits represent the category and number of the book. A book number is a digital code used to identify and track books. It allows both the author and the reader to easily identify and find different versions of the same book or different publishing houses. In e-commerce, the number was also widely used to track the sales and inventory of books.