According to the documentation at https://portableapps.com/manuals/PortableApps.comLauncher/ref/envsub.html, %PortableApps.comXXXX% environment variables should point to %PAL:PortableAppsBaseDir%/Documents.
The PortableApps.com Documents directory, normally X:\Documents. There is no guarrantee that this directory will exist.
The base of the PortableApps.com directory hierachy, where PAL:PortableAppsDir and PortableApps.comDocuments are usually at.
However, since at least 2015 (see my comment https://portableapps.com/comment/224987#comment-224987), The %PortableApps.comDocuments% variable points elsewhere, namely, "%PAL:DriveLetter%\Documents". This fact was noted on the following thread (https://portableapps.com/node/43994). The first comment, by Gord Caswell, says:
The PortableApps.comDocuments variable, by design, is hard-coded to reside in the same directory as your PortableApps directory
I agree that's what should happen, but that is not what's happening. I cannot follow the thread much further than that, as it gets into NSIS code, with which I am unfamiliar. But it is clear from the behavior that the *expected* value does not match the actual.
I suspect few people care about this because the PortableApps - Documents directory is so rarely used, which is a shame, really.