Every item in the library has its own address, the call number. For non-fiction, the next part is the DDC classification number that places the item with other items on the same topic. If there is more than one edition, the year may be added, and sometimes a volume number is needed. The call number is part of the catalog record and is also marked on the item or attached as a label.
People can understand the call numbers, but automated systems use bar codes instead.
So bar codes are also created as part of the process of preparing library materials to go on the shelf. Look at the example below. Technical services people prepare materials for users. Public services connect users with library materials. I.American Library Association.