...
When a publisher creates an "edition" that deserves mention in metadata as an Edition they've changed the content of something in one of their published books in some substantive way and issued it under a new ISBN. There's an old product and a new and likely improved product. In a few special cases Edition is used to show some special market or limit in the supply chain: "Large Type" or "Ultra Large Type" are considered "Edition Codes" in metadata (a special market) and so are books that are published initially as "digital editions" without any print counterpart (atypical supply).
Let's start with the typical case in publishing: All formats – hardcover, paperback, digital, etc. -- that share the same text or text-and-illustration combination are "identical content" and are considered to be of the same "Edition." Or to state that another way: By far the vast majority of books published come out in a single original edition and are only available in multiple formats. In good ONIX metadata these are identified by the <NoEdition /> tag which very accurately says what it means: Don't go looking for "edition" related information about this book because it doesn't exist. You can improve the metadata entry by providing a "Work Identifier" that shows all these entries are, indeed, the same content – and the "persistent" identifier to improve discoverability would should be something called an ISTC, but I'm afraid it hasn't yet proved workable (we know we need but won't support it...). All publishers should use at least a proprietary Work Identifier and if you must use an ISBN (usually of the first released product) then be sure to use that ISBN in ALL of the products including the book you take it from. End users should be able to simply "select" using the Work Identifier and get all the books.
Let's pause for a moment and think just how useful <NoEdition /> is to end users: A marker for "All the formats I sell share the same content!!" Genius -- thank you EDItEUR! That means there's no extra implication here for Related Products beyond those listed above.
That takes us to the most typical metadata use for edition information which is using an "Edition Number" to mark a later edition. The general convention is that a sequential Edition Number is used when you published an updated book where 20% or more of the content is changed or new. It's a marker to show libraries and previous buyers that they should consider replacing their book copy, and for retailers so that they know that this is the book they should be offering for sale. Lesser changes than typical for an Edition Number but that still warrant a new ISBN can be marked using a "Edition Type Code" that describes the change. It really does NOT make sense to do both at the same time. An edition number "2" combined with "Revised" implies that there may be an edition number "2" prior to this "Revised" version so it's very much an either or choice (unless you really are revising the second edition and issuing it as a new ISBN – and then I have to wonder why it's not "3" if consistency is key).
...