The original AZW format was MOBI-based, not PDF-based. MOBI originally from a company called MobiPocket, which Amazon eventually acquired, was built to be an ePub competitor and like ePub was an HTML and JS-based solution, but in a somewhat different, proprietary DRM-friendlier container format. (ePub is "just" a ZIP file, with the DRM applied sometimes inside the container rather than outside it.)

MOBI stopped keeping up with ePub standards and standard features, in part because Amazon acquired MobiPocket. The KFX is just ePub with a new proprietary DRM container around the ZIP file that is ePub's container.

The 2013 boundary is also the "supports ePUB files directly without a conversion process" boundary in Amazon's kindle OS. It's not just useful to know for book file authors, but as a consumer it becomes useful for a quick "Can I buy a standards compliant DRM-free EPUBs such as from sites like DriveThruFiction and just send them to my Kindle with no other steps?"

▲

chocochunks 7 hours ago | parent [-]

No Kindle supports ePub natively. Amazon converts ePub to a supported format when you use the send to kindle email service. If you just load the book on over USB it won't work.

▲

WorldMaker 6 hours ago | parent [-]

Every kindle that supports the new format (Kindle devices since 2013 with latest OS upgraded) support loading non-DRM ePubs directly over USB. There's no conversion anymore. (I've done this.)

Amazon's not going to openly advertise that this deprecation is also the line in the sand where "non-DRM ePub just works", but that's what has happened.

Of course one of the sadder problems with the ePub ecosystem is that it uses the same file extension for DRM contained and non-DRM contained ePubs. At a glance it isn't easy to tell if an ePub is not DRMed. Amazon does not support any of the existing ePub DRM schemes. Their own KFX DRM is very unique and proprietary and doesn't play nice with ePub DRM "standards". You can't load DRMed ePubs over USB, those don't work. Sometimes that gives an impression still that "Amazon does not support ePubs natively", but that's the nature of DRM and how much DRM hurts the entire ebook industry in every direction.

▲

chocochunks 4 hours ago | parent [-]

Are you sure about that? Even Amazon's own sales page state: "Kindle Format 8 (AZW3), Kindle (AZW), TXT, PDF, unprotected MOBI, PRC natively; PDF, DOCX, DOC, HTML, EPUB, TXT, RTF, JPEG, GIF, PNG, BMP through conversion; Audible audio format (AAX). Learn more about supported file types for personal documents." implying that ePub only works through conversion. They don't support DRMed ePubs through conversion either so it's a bit odd they say that instead of including it natively.

	▲	WorldMaker 4 hours ago \| parent [-]
		As I said, anecdotally I've already done it. Amazon only just enabled the PC "Send to Kindle" to support ePub directly instead of the old silly work around of rename the .epub to .kfx (and no other change). They've been very bad at keeping their list of formats up to date in their own documentation. Some of that perhaps because they don't want it to be so obvious and it is intentional obfuscation (to keep people using their store rather than going elsewhere for books), some of that because a lot of their kindle documentation seems to be in a "isn't broke, don't fix it" frozen state for years at time. You'll also note that the text you found doesn't mention "Kindle Format 10 (KFX)" at all and also you might notice that TXT and PDF are mentioned on both sides of that text as both "natively" and "through conversion" which seems to imply the original text was from the era when they were converted and they were added to the "natively" side later without remembering to clean up the other side. (They both have native support today.)