Calibre on its own can't read encrypted books. The deDRM tools will decrypt the books and make them readable.
I did a
bit more reading, and AZW3 is a file format that contains a file that includes both Amazon's newest formatted file as well as the old MOBI formatted. I hope I'm saying that right. It also said that it's Amazon's newest format, so it includes some of the newest code (like HTML5).
There are certain books - often children's books with multimedia formats - that can only be read in certain programs.
I don't have any of those, so I've never bothered to find out if Calibre can't read it.
My rule of thumb is, if I import a book to Calibre and it won't display the book and instead of a book cover it's just a graphic of a book, it's probably encrypted and I need to strip off the DRM for Calibre to read it.
Once DRM is stripped, the file has the same extension (so the AZW3 stays an AZW3), but is no longer encrypted and can be read by Calibre. It *is* possible there are certain multimedia ebooks that can't be - I haven't researched or experimented with any of those.