Does anyone know the difference between 'Operational Control', where Bulow of 2nd Army had operational control of 1st Army as Senior General on the Right wing and 6th Army had operational control of 7th Army on the Left Wing, and an 'Army Group - HeeresGruppe' where Mackensen has is own 11th Army and the Austro-Hungarian 4th Army?
I suspect the difference is pedantic, but therein lies its interest.