[Quoth]

You need an integer?
0 = epub2
1 = basic epub3, a reflowable ebook, no interactive and fall backs for epub2 (probably Project Gutenberg epub3 titles).
2 = basic epub3, but no epub 2 index etc, no fallbacks, specific hand crafted epub3 inc math ml. No javascript, transparency, animation, audio or video.
3 = FXL aka Fixed Layout aka Print Replica epub3 imitating a PDF.
4 = Multimedia and/or Interactive epub3. May have javascript, video, audio etc.

Obviously given epub 3 flexibility and features it's more complex. Maybe also the epub3 numbers should start at 2 and 1 should be for Kobo's kepub.

Any reflowable mobi, prc, docx, odt, lit, pdb, lrf, epub2, azw3, kfx or kepub converted automatically with no hand editing is going to be a type 1 epub3.

However these are the broad categories and it's what I would do, if I wanted it.