From Halopedia, the Halo wiki
A service number is a numerical designation given to each member of the United Nations Space Command as a means of identification.
Service numbers usually follow a ten-digit format followed by two letters that are sometimes the first and last initials of a person. There is no known exact format for service numbers, as some follow this format while others do not. Service numbers are used in conjunction with the UNSCMID though the former is far more widely utilized and their exact relation is unclear.
All soldiers of the Spartan-II and Spartan-III Programs had nonstandard service numbers, such as S-117 or S-B312. However, Spartan-IVs kept their standard service numbers when joining the Program.
Colonial Military Administration servicemen were also assigned UNSC service numbers. These numbers followed the standard ten digit format, though in every known case the two letters did not match the individual's initials.
Known service numbers
Civilian Consultant numbers