The KF8 format is currently only used on the Kindle Fire and (perhaps, reported to work - but undocumented) on Kindle for PC.
It's not only purported to work on Kindle for PC and Kindle for Mac (>= v 1.9.0), it does
work. And the prompt that comes up asking you if want to upgrade to the latest version says as much.
Re-downloading my past purchases from my Kindle for PC archive reveals many stand-alone KF8 books. The problem is... they look like a MOBI, smell like a MOBI and act like a MOBI (because they're still in a prc container). So unless people know enough to dump the headers of the files and actually check, there's just no way to know for sure.
Which means that a very common workflow is going to result in DRM-free MOBI's being imported into calibre libraries that are, in fact, stand-alone KF8's that calibre can't view properly and can't convert to anything else.
It's going to result in some confusion for a while.
Originally Posted by dwig
KF8 files generally contain two versions of the formatted text, one for KF8 compliant readers and one for non-KF8 compliant readers. Both versions need to be tested.
That's only the version that you upload
to Amazon. Amazon is splitting the MOBI-only portion from the KF8 portion and delivering the appropriate file to the various devices/apps depending on what they can support. The KF8 ebooks that are being delivered to my Kindle for PC have no fall-back format. They simply can't be viewed at all
on the eInk devices (I'm talking about DRM-free versions, of course).
I agree that MOBI is still going to be the more common format for quite some time, but KF8 is no longer a "red-headed stepchild." It's out there... people just don't know it yet.