According to the documentation at https://portableapps.com/manuals/PortableApps.comLauncher/ref/envsub.html, %PortableApps.comXXXX% environment variables should point to %PAL:PortableAppsBaseDir%/Documents.
PortableApps.comDocuments
The PortableApps.com Documents directory, normally X:\Documents. There is no guarrantee that this directory will exist.PAL:PortableAppsBaseDir
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.
The Documents variable is dictated by the platform, not PAL. PAL's documentation hasn't been updated. If I recall, at present, it defaults to X:\Documents if that directory exists otherwise it uses the one alongside the PortableApps directory.
Sometimes, the impossible can become possible, if you're awesome!
Currently, PAL uses the Documents variable if set by the platform.
If that variable is not set, it defaults to X:\Documents.
In an upcoming release of PAL the logic is a bit more complex: