Where can I find the APFS format documentation / volume specification?

I think Alsoft's use of the phrase APFS format documentation is just that - a phrase they've used rather than referencing an official title.

The Apple File System Reference is almost certainly what they're referring to. This reference was publicly released on 17 September 2018, and Alsoft made that statement on 4 October 2018.

Prior to the Apple File System Reference, there was no consolidated documentation from Apple regarding APFS - at least not to this level of detail. In fact, the most comprehensive documentation until this was released was actually the result of private academic work in the computer forensics field.

As for Apple itself, we've had the following documentation:

• Apple File System Guide - retired in June 2018
• About Apple File System - first published in June 2018, but updated in October 2018 to replace references to HFS+ with HFS Plus instead
• File System Programming Guide - last updated in April 2018

However, the Apple File System Reference was last updated in February 2019 June 2020.

Apple has not yet published full APFS documentation. Only information about reading APFS disks has been released. Missing documentation is about how to write to APFS disks. Until then, DiskWarrior 6 to rebuild APFS disk directory cannot be released.

Check out what Micromat says about it in relation to TechTool Pro:

APFS - AGAIN no support

Providing full support for APFS is entirely dependent on Apple releasing the required documentation for developers. So far, only the documentation explaining the APIs for reading APFS volumes has been released. It is about 140 pages long. We need similar documentation for writing to APFS volumes.

^{source: https://www.micromat.com/component/kunena/techtool-pro-11/5225-apfs-again-no-support}

Why? When will it happen? Surprisingly, APFS is still a work in progress. We could say that it is still in beta phase. Hopefully, Apple will announce Time Machine 2 supporting APFS on June 2020 (WWDC). Then, macOS 10.16 with it will be released by end of September 2020. And then Apple will hopefully release really and truly full (sort of) APFS documentation. Then DiskWarrior 6 could be expected in about three to six months testing period. Hopefully.