As I understand the basic settings in Calibre for Kindle is Mobi. Because AZW3 support is still in beta.
However at
http://manual.calibre-ebook.com/conversion.html
I read:
To edit ebooks, I recommend first converting them to EPUB or AZW3 using calibre and then using the Edit Book feature to get them into perfect shape.
And:
There is a demo .docx file that demonstrates the capabilities of the calibre conversion engine. Just download it and convert it to EPUB or AZW3 to see what calibre can do.
So you get it. Mobi is old. But still the default because AZW3 is in beta. OK, I get it, but still a bit confused.
By the way, for my interest, I understand Epub is an open format and AZW3 is closed but how closed? Can you get a code or documentation on how to produce it? Or how Calibre produces it if it is so proprietary to Amazon?