From Halopedia, the Halo wiki
A commission is a formal legal document that grants authority to a commissioned officer in a military. In the Unified Earth Government, a commission is issued on the authority of the President of the Unified Earth Government.
Within the UNSC Armed Forces, a commission is awarded to personnel after the completion of Officer Candidate School and after taking the officer's oath of service. Warrant officers are given a commission and take an oath of service in recognition of their expertise and leadership role, though they are subordinate to commissioned officers. All operatives of Spartan Operations receive a direct commission as an officer in the Armed Forces, typically equally the authority of a first lieutenant, though their authority does not typically extend outside their chain of command.