The file format version or FFV number was part of the original MOBI format created by Mobipocket and purchased by Amazon. It provides a way for reader software to know whether or not it is capable of properly interpreting the content of a particular book. The version number was incremented each time new features were added to MOBI format.
Books are not simply assigned the latest version number. When a book was converted to MOBI the particular features present in that book were used to determine the version number assigned to it. That way old reader software could be used for newer books as long as those books didn't use any recently added features.
As far as I know even the oldest Kindle is capable of handling FF version 7 and so version numbers 7 or less are effectively treated the same. Despite that Amazon's kindlegen software still produces the correct version number for MOBI format based on the features it contains, so newly created MOBI files can still have a version number less than 7.
I haver never seen a complete list of features are associated with each version. This is the best I have come up with so far:
- 0-2 I have never encountered MOBI files with these version numbers.
- 3 Documented as indexes for images
- 4 EXTH, FCIS, FLIS, orthographic index, names index
- 5 RESC, multi-byte overlap trailing bytes in content records (Mobipocket Reader 4.8 - 2005)
- 6 NCX index, multiple flows, non-overlap trailing bytes in content records, DRM V2, always produced by calibre for MOBI
- 7 audio, video, UTF-16 Unicode in indexes, ingestion by kindletool (prcgen/mobigen v5.0, Mobipocket Reader 5 - 2006)
- 8 KF8 based on HTML5 and CSS3
- 9+ No V9 was ever released and KFX (Kindle format 10) is a totally different format