Hope this helps:
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KF8 (also called AZW3) is basically a compiled ePub that has been compiled using a Palm database and Amazon's DRM scheme. It is targeting ePub 3 version support but existing ePub can also be used. The latest KindleGen software will create KF8. KF8 replaces MOBI but actually includes both a MOBI database and a KF8 database in the same file for backwards compatibility using older Amazon Kindle readers. ... The internal format has been decoded using kindle unpack. It reveals that the basic format remains as a PDB file similar in structure to MOBI. Generally a KF8 file may contain both a MOBI file at the beginning and the newer KF8 version of ePub later. This of course increased the size of the book file although there is some attempt to share resources such as images between the two objects. Kindle Unpack can be used to separate the two structures by building a traditional MOBI file and a KF8 file without the MOBI part (actually a small dummy structure remains) to make the file smaller. There are flags near the start of the database that can be used to identify the type of file. In spite of the fact that KF8 is targeted at ePub 3 source it still carries forward some HTML constructions that have been de-standardized for years and left over from MOBI's use. Amazon does publish the HTML statements and CSS3 statements that it will recognize. In addition KF8 has extended the ePub 3 format in incompatible ways to support its fixed layout option. All Kindle products beginning with the Kindle 3 can support KF8, although the Kindle 3 requires an update to do so.
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Kindle unpack (originally called Mobi unpack), also known as Mobi decode, is a python script that creates MOBI or AZW (Amazon Kindle) source files from the compiled database. The filenames used in the source file are not necessarily the same as those that were originally used to create the database as this information is not preserved in the database but an unpacked set of files should be able to be used to recreate the same database using standard mobi or Kindle generating tools. For KF8 files and combined Mobipocket and KF8 files built by KindleGen, it also can produce separated mobipocket and KF8 files, and also the original source files if those are included in the eBook. In addition, for KF8 files it can produce an 'ePub', although if the HTML isn't compliant with ePub standards, the 'ePub' won't be either. For Amazon's .azw4 files, it will extract the PDF that's been wrapped up in Amazon's .azw4 file format. A Calibre plugin version of the scripts is available in this thread.
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The trailing 'r' on the file extension means it is a "sidecar" (metadata) file, which in this case may contain your added notes. Here is a related mobileread discussion regarding sidecar (including azw3r) files:
https://www.mobileread.com/forums/sho...d.php?t=262356