Welcome to the Metadata Millennium: A Complete Overview of What Metadata Can Do for Publishers
Identifiers: Eliminating Ambiguity
When content is released "into the wild" of the web, it's critical to be able to identify things that may have seemed perfectly clear within the publisher's walled garden but are ambiguous outside it. The web has URIs (Universal Resource Identifiers, of which the familiar URLs are a subset) that make it possible to identify and link things on the web, but publishers use many other identifiers in the context of producing and selling their books. Those identifiers are a specialized form of metadata (and the identifiers themselves are often associated with specific metadata that describes what they identify).
The most familiar of these identifiers, of course, is the ISBN (International Standard Book Number). But despite what its name implies, an ISBN doesn't identify "the book" as the publisher thinks of it. Instead, it is a product identifier, designed for the supply chain to enable the precise identification of a specific product. Contrary to what publishers often think, there is no such thing as an "eISBN": each distinct ebook version -- EPUB, Mobi, PDF, etc. -- needs a distinct ISBN, just as the hardcover, paperback, and audiobook do, because if somebody wants to buy the EPUB they don't want to get the PDF instead.