Between 1968 and 2011, the U.S. military used an individual's Social Security number (SSN) to identify the serviceman. Now, though, the Department of Defense will use a unique 10-digit number to identify each member. That number, the federal identifier, will correspond to the department in which the person works.
Note the number of digits in the number. This can indicate the time of service and sometimes the branch of service. For example, a nine-digit number indicates armed service between 1968 and today, while an eight-digit number indicates Army or Air Force service between World War II and the Vietnam War.
Note the first one to three digits of the number, depending on how many digits the number has. This helps indicate the branch of armed service. It might also indicate where the member is from, or the speciality or assignment of the service member. For example, an eight-digit number beginning with 13 belongs to a regular (not drafted) Army or Air Force member from between World War II and Vietnam who enlisted in Maryland, Pennsylvania or Virginia.
Look for a prefix or suffix to the number, or a military occupational speciality (MOS) code. This indicates what the service member did during his time in the military. Such codes are in less frequent use now, so they indicate service during an earlier period. For example, the suffix "FR" appears after a present-day service number only in the case of regular Air Force members.
Compare the service number to a publicly available record, such as those maintained by the National Archives. This will indicate the precise person to whom the number was assigned.