I just saw a user create an Edition Group based on geographical location of publication, i.e.: US editions, and European Editions.
According to the help page a logical grouping of Edition Groups is by format, e.g. hardcover, paperback, eBook: https://bookbrainz.org/help
I’m wondering if someone can create an Edition Group based on any criterium. To give an extreme example, all the editions of Pride and Prejudice that have a black cover?
Addendum: the more I thought about this it only makes sense to create the Edition Group for the type of format. The Edition can only link to one Edition Group and if users are creating them for purposes other than the format then the system falls in a heap.
The word “logical” is very subjective!