What makes you think AZW3 is a beta format?
I believe the reason it is default is because it is still relatively new -- older Kindles which many people are still using don't support it.
The MobileRead wiki has useful information regarding
MOBI, also you can take a look at the
KindleUnpack plugin. Or examine calibre's source code:
https://github.com/kovidgoyal/calibr...re/ebooks/mobi
All current work on decoding AZW3/MOBI has been done in python.