I've searched thru InfoHub (and SupportNet) and cannot find any reference to this "NET_SPACING_HR" entry.
I've also look thru DxDesigner Support Files and Variables > DxDesigner Support Files > DxDesigner.xml File manual and can find the NET_SPACING defintion but not the NET_SPACING_HR definition.
Funny thing about the InfoHub search for NET_SPACING... it can't find this string when I select a local scope of "All EE 7.9.2 Installed Documents", yet I can manualy find it in "DxDesigner Support Files and Variables > DxDesigner Support Files > DxDesigner.xml" section
The _HR is the newer format and refers to "High Resolution" or 2540 times greater than the original inch based format.
NET_SPACING would be the default spacing between nets as defined in by DxDesigner's precursor tool. Today the spacing is determined by the current grid spacing.