Here's what Wikipedia has on the ISBN:
ISBN
Quote:
An ISBN is assigned to each edition and variation (except reprintings) of a book. For example, an ebook, a paperback, and a hardcover edition of the same book would each have a different ISBN.
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So if your book is hard cover you will have one for that, another if you go to soft cover and another probably for ebook. That way publishers can tell which edition/variation of your book they are dealing with.