From Halopedia, the Halo wiki
Private First Class, abbreviated to PFC, is a Enlistment rank of E-2 in the United Nations Space Command Unified Ground Command (UNICOM) rank structure. It is preceded by Private and subordinate to a Lance Corporal.[1]
Initial assignment.[1]
Enlisted: Crewman Recruit • Crewman Apprentice • Crewman • Petty Officer Third Class • Petty Officer Second Class • Petty Officer First Class • Chief Petty Officer • Senior Chief Petty Officer • Master Chief Petty Officer
Enlisted: Private • Private First Class • Lance Corporal • Corporal • Sergeant • Staff Sergeant • Gunnery Sergeant • Master Sergeant • First Sergeant • Master Gunnery Sergeant • Sergeant Major