To ensure innovation in military behavior (IE Thorne's idea to hijack Covie vehichles), all servicemen and servicewomen apart of the SPARTAN branch have the exact same rank, which is Spartan. In order to allow maximum functionality, the Spartan branch does use series of titles to create the semblance of chain of command. So for Sarah Palmer, she would be called: Commander Spartan Palmer. The 'Commander' is a title, and the 'Spartan' is her rank.