Where and how is the variable _USTN_WORKSETDGNWS defined?

I am trying to configure ORD CE for ProjectWise.

Our folder structure does not support worksets under workspaces but there a videos on how to configure that. I have been successful in making out projects work as worksets and associating them with a workspace. However, my efforts to enable the sheet index (and whatever else a DGNWS file provides)have failed. I have defined a common location for the Workset CFG files and location where the DGNWS file should be created using _USTN_WORKSETSDGNWSROOT. 

But I cannot figure out how to define  _USTN_WORKSETDGNWS in PW. According to its description, it should be the full path file name of the DGNWS file. But in my PW Managed Workspace, it is being defined to the Workset name only and according to the Configuration Dialog box, it is being set at a System Level, While most of my workset variables at reported to be correctly set to Workset Level.

I discovered that if I list the Configuration Variables using the Show Configuration Variables command of the dialog box and compare its list to the Summary of Variables from an MSDEBUG.TXT file, there are a number of assignments that are missing. Particularly glaring is the _USTN_WORKSETDGNWS assignment. In fact, I have searched every CFG I could find and do not see it listed anywhere.

If it is automatically generated, are there rules and conditions that can effect its assignment? I need to figure out why I cannot get the variable and its operational purpose to function in Project Wise.