Hi MTP,
I’ve heard back from our head of metadata programs.
So, a hardback vs. paperback vs. html vs. pdf are considered different formats, whereas a different edition of a book could be a version. Further, an updated copy of a digital text that isn’t big enough to be considered a new ‘edition’ would be a variant. Specifically, the difference is a ‘variant’ has a difference between the text, but often it’s not enough to consider it to be a full ‘version’.
I hope this helps. Do let us know if you have more questions.
Evans - Crossref Tech Support Specialist