Starting at the basics, we have ships. Ships are comprised of its crew, SFC's, Aegis Infantry (Marines), shuttles, Heavy Combat Gear, and other types of Vehicles and personnel.
Task Forces are lead by Task Force Commanders and are comprised of 13 ships. 8 are Misc ships from the base of the fleet, 4 are Support Ships (Ordinance, Medical, Experimental, supply or Special) and the High-Command (HiComm) Carrier. The 8 ships can be of any size and my include the support ships. Task Forces are generally geared towards areas of expertise and will be configured to accommodate those purposes.
Fleet Forces are lead by Fleet Force Commanders and are made up of 4 Task Forces and a Major-Command (MajComm) Carrier. The MajComm carrier is usually larger than the HiComm carriers. Fleet Forces are configured with Diversity in mind for the area of operations with its operating theater details taken into account (Environment, history, etc.) There are several Fleet Forces in Operation where its Task Forces are all configured for Combat.
Fleet Forces are Assigned to Stations where another Admiral oversee's the operations of the Fleet Forces and their Task Forces. Stations operate under the direction of the Fleet Admiral.
The Fleet of USI is Commanded by Fleet Admiral Kealan Sells.
The USI Star Corps
Every Fleet Force and the Separate Division of USI operate under the USI Star Corps. The USI Star Corps is Commanded by the Major Admiral, Currently Major Admiral Kerok Seccour.