ISBN
A book edition's unique international identifier (13 digits) — each format gets its own.
The ISBN (International Standard Book Number) is a book edition's unique international identifier: thirteen digits encoding the prefix (978/979), the language area, the publisher and the title. A decisive, often ignored point: the ISBN identifies an edition, not a work — hardcover, paperback, ebook and audio of the same text each carry their own number.
It's what makes a book orderable: distributor, bookshop and library referencing. ISBNs are obtained from each country's agency (free in some countries, paid in others) — and the savvy self-publisher prefers owning their ISBNs to using a platform's: the number states who the book's publisher is.
Example
The same novel: one ISBN for the hardcover, another for the paperback, a third for the ebook.