Theory (debunked):

Alex made Quest work with XML. He needed a file extension which could be set up to be associated with Quest, though, and he couldn't really do that with the XML file extension, so he decided on changing the XML to Alex's Specification Language. BUT, Photoshop uses ASL, so he just tacked on an X and rolled with it.

Are you sure? I think the previous version of Quest (1 to 4 in fact) used ASL. The X was added for XML, I guess. It is possible the A stands for Axe. Some clues on this very old page here:
http://www.firthworks.com/roger/cloak/quest/index.html

Are you sure?

Not at all. That was just a wild theory.

Ah!

The ADVENTURE SCRIPTING LANGUAGE!

That is much cooler!!!

Thanks, Pix!

I recently found this stuff:

The History of Quest

https://blog.textadventures.co.uk/2013/11/07/quest-is-15/

https://blog.textadventures.co.uk/2013/11/26/15-years-of-quest-part-2-1999-2000/

https://blog.textadventures.co.uk/2013/11/27/15-years-of-quest-part-3-2000-2004-experimenting-with-multi-player/

Wow, that's great material! Perhaps there should be a "History of Quest" link on the home page? ...updated as developments occur?

Whoa... I didn't even watch that video (or really read the title)...

I removed the link. I'll let people find that on their own. It doesn't seem to... um... fit.

...my suggestion was in connection with the blog material? ...the video is perhaps the basis of a separate thread...

Oh, sorry, DB! I removed the link to the video after I actually watched it.