From - https://isbn-information.com/index.html:
“There is no requirement to assign an ISBN to any book, but a publisher will find that for all practical purposes it is necessary to adopt the ISBN system for its books to be adopted by the book trade.”
“The ISBN system is recognised in over 170 countries, regions or language groups throughout the world.” -
“ISBNs are not just used for print media, but identify a range of media types including audio books on cassette, CD and DVD, as well as other electronic media;[…] educational computer software as well as publications that consist of a range of media types, where the component is text based”
“Publications unsuitable for the ISBN system include those that are ongoing and unconcluded, such as bibliographic resources, newspapers and periodicals; the abstract intellectual property of a work, such as a novel (while a particular publication of the novel would have an ISBN); printed ephemera such as advertising leaflets; printed music; art prints without text; personal resumes and curriculum vitae; greetings cards; recordings of music; non-educational software; emails or other electronic communication.”
“Other systems exist for publications that do not come within the range of the ISBN system. These include the ISSN (International Standard Serial Number) used for magazines, periodicals and journals and the ISMN (International Standard Music Number) used for printed music.”