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I can see ZIP not being a good option as Calibre normally uses the ZIP extension for storing HTML books (which can consist of multiple files). I have seen ZAB mentioned as a common extension for ZIP'ed audiobooks so maybe that would be a good one to use?
Not quite sure why RAR should not be used unless it is the fact that Calibre peeks inside that format to see if it contains a recognisable eBook and if so extracts it before adding it to the Calibre library.
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