The International ISBN Agency, and its national agencies such as Bowker in the U.S., have repeatedly insisted that the correct use of the ISBN system requires each format of an ebook be identified with its own ISBN, as is the case with print books. Thus a Kindle file should have its own ISBN, the ePub its own too, a PDF yet its own, and so on. However, some indie authors who have purchased their ISBNs from Bowker in the U.S. insist they can assign a single ISBN to cover all digital formats because of an option in the Bowker database to select “multiple formats” for “file type” after selecting “ebook” as the medium and “electronic book text” as the format.
Confused, I put the question to the International Agency who informed me this is an incorrect interpretation of the option and referred me to the ISBN Users’ Manual. The “multiple formats” option exists only for digital files sold as an archive; that is, a single file that contains within it multiple digital formats. An example of this would be an ebook file that, once downloaded, gives the consumer the choice to select a mobi, ePub, or PDF file to open in their ereader; this would be similar to the way online software manuals are often provided as a single file download that, when you open it, gives you the further choice to select an English-language file or a Spanish-language one, and so on.
There is no print equivalent to such a file; the closest comparison would be to a multi-volume set, such as an encyclopedia, where individual volumes are not sold separately. As Nick Woods, Operations Manager for the International ISBN Agency, put it to me in his email:
Perhaps a good comparison for the single digital archive arrangement is a ‘bundle’ whereby a set of printed books is sold as a complete set (section 5.6 [of the ISBN Users’ Manual]). As you can see in the manual, a single ISBN can be assigned to cover a multi-volume set but only on the strict condition that the individual publications in that set are not available separately. As the manual indicates, however, the International ISBN Agency would still recommend that the individual publications are also allocated separate ISBNs.
Therefore in answer to your final question, if these self-publishers are intending to sell each one of their multiple [ebook] formats independently then they are using the system incorrectly. The multiple formats option exists to allow the sale and distribution of multiple formats packaged together only.
Where the confusion has arisen, I believe, is that none of the major retailers we deal with sell multiple-format ebook files so our familiarity with such a file is negligible, yet the multiple formats option exists for us to choose from when purchasing an ISBN for our ebooks. I can therefore see why some indie authors would mistakenly believe that applying a single ISBN to multiple formats sold individually is not an incorrect use of the system. Nevertheless, now we know, so if you have mistakenly assigned a single ISBN to multiple formats of your ebook you may wish to fix your records.