The first is the classification number. Different libraries will use different classification methods to classify books. Most libraries in China use the China Library Classification, as well as the classification of Ancient Map and NPC. In addition to the library classification number, the author number and other classification numbers should be added, that is, the classification number is the first. If the classification number is the same, it should be divided by the author number. If the author number is the same, it should be the version number. Popular points are arranged in the order of theme, author and version.