Originally Posted by GoSharks
In Calibre when you convert you have the option of creating a standard format mobi file (old), an azw3 format file (new), or both formats included in the file (both) this last category also nearly doubles the size of the file for obvious reasons. I'm not the best to explain the difference, but my understanding is that the 'old' format can be viewed on all kindles, the azw3 'new' format (also called KF8) has more expansive font and formating abilities but can only be read on newer generation kindles, and that when your device tries to read a file containing both, it will pick the one best suited to it. I also understand that Amazon does not support emailing of azw3 formats at this time.
As I have an older kindle (and a Fire) and don't really care about special formating in my books, I convert my books to the 'old' format.
I hope this helps. If something is inaccurate in what I said, I'd love someone to clarify.
Not to mention the new format has that line bug. I got a book the other day downloaded it to the Kindle and the line spacing was like 3 lines on even the smallest settings.
I really hope Amazon fixes this someday.