After the capture of Rostov, Hitler decided to split up Army Group South in two smaller army groups.

Hitler's Directive #45

Army Group A : 
Commanded by Generalfeldmarschall List

Army Group B : 
Commanded by Generalaoberst Freiherr von Weichs.

Army Group A was ordered to capture the Caucasus.
Army Group B was ordered to reach the Volga and capture Stalingrad and Astrackan.

The German 6th Army was part of Army Group B