ISSN or ISBN
Choosing between ISSN and ISBN depends on the type of publication you're working with. If you are publishing a serial (a periodical that is issued in multiple volumes or issues, like a journal, magazine, or newsletter), you will need an ISSN (International Standard Serial Number). The ISSN is used for serials published over time, whether in print or online formats. If you're publishing a book or a standalone publication (like an eBook or paperback), then you will need an ISBN (International Standard Book Number). The ISBN uniquely identifies a specific edition or version of a book. In short, ISSN is for serials, while ISBN is for books.
