$(_DGNLIB) in Parts file

I'm getting some errors in the message center when opening a new file.

Could not find material definition $(_dgnlib)|project_materials.dgnlib|_materials.pal : matl_mirror assigned to part: 79 furnishing fixed - mirror

The first thing that occurs to me is that there are a couple of those definitions in the parts .xml files, but I can't see that _dgnlib is defined in our configuration.

So the question is, why are those definitions in the standard workspaces (_DK and _NM for example), and how do they work?