A clone marshall commander, such as Cody, were clone trooper commanders elevated to the rank of clone marshall commander and placed in command of a corps level formation in the GAR. As such, they would be the equivalent of a major general or lieutenant general in other services.
They would be allowed latitude in strategy and tactics when planning battles and formations, as well as recieve little oversight when enacting these strategies.
A few notable Clone Marshall Commanders are :Commander Bly of the 327th Star Corps , Commander Cody of the 7th Sky Corps, Commander Bacara of the 21st Nova Corps(which contains the Galactic Marines), Commander Neyo of the 91st Reconnaissance Corps, and Commander Appo of the 501st Legion.
This information was referenced from Wookieepedia and the "Guide to the Grand Army of the Republic" article by Karen Traviss and Ryan Kaufman printed in SW Insider 84.