Supreme Commander was a rank given to the senior-most officer in Imperial High Command. Although Emperor Palpatine was the overall leader of the Imperial Military as Head of State and was advised by the Joint Chiefs, the Emperor was not involved in the daily responsibilities of the military-industrial complex. Those responsibilities fell to the Supreme Commander, who was briefed daily by the Joint Chiefs, created the agenda of the military to fulfill the Emperor's galactic strategy, and led High Command.
The individuals who achieved the rank of Supreme Commander reportedly did not attain it for long, as the Emperor promoted infighting and competitiveness among the senior officers, and shifted assignments and responsibilities frequently.
- Dawn of Rebellion (First mentioned)