Hi,
I suspect the reason that NFS doesn't publish a list of pound variables is because they are internal to Mach and subject to change.
Using an API call as Daz has suggested will undoubtedly actually refer to the same internal register. If however in a later release of Mach where the pound variable number has
changed the API call will still work.
Using pound variables became common in Mach3 because you could use them inline Gcode, for example:
G1 X(#500) Y(#501) Z(#503)
whereas using OEMdro(*********) required it be contained in a macro.
Mach4 is the same, you can use pound variables inline but have to call a macro, a Lua program, if you wish to use API's.
Craig