Basically true. However, if you use the latest version of Kindle Previewer to compile a book you will wind up with both formats. The thought being, I assume, is that a single file can be downloaded to various Kindles and/or devices running the Kindle App and this single file will run on all of them. As we know, not all devices and apps support Kindle KF8 formatting, so this ensures that a KF8 book can still be read on a Kindle or Kindle app that doesn't support KF8. That might sound strange, but it prevents the issues that occurred when Sony switched to ePub from BBeB. No need for two files for the same book. If you have two different files on a kindle, you cannot easily (if at all) Whispersync them between devices. In theory the single file with both formats allows the book to be Whispersynced across devices where some support KF8 and some do not. Well, at least that is the way I think it is supposed to work.