In Imperial Service Edit
A Major General commands a Corps, consisting of several battlegroups. In practice the Corps is one of the most important levels of organization in the Imperial Army. Cooperation with the Navy is best at this level of command, and the Army's standard transport ships are built to accommodate a Corps each.
In New Republic Service Edit
A Major General in the New Republic Army would serve as commander for an entire Corps. They were brass officers, coordinating efforts in with the Director of Ground Operations, and occasionally acted as combat operation commanders.
In the New Republic Starfighter Corps, these men serve as Wing Commanders. Also brass officers, they served as direct aides to the Director of Starfighter Operations.