Army and Corps Commands

[Enfield]

Based on sourcebooks like the US Army Vehicle Sourcebook, we have organizations like:

9th US Army, 8th US Corps, 6th US Army, still appear to be active according to Howling Wilderness as well.

I'm curious about what they'd be doing. I imagine them having a headquarters somewhere, perhaps issuing orders to units that no longer exist. Or would they be attached to an operational division?