Military Occupational Specialty, abbreviated "MOS", is assigned by the United States Army to its personnel. It denotes the extra training and duties that Soldiers have. Enlisted personnel can choose their MOS upon initial enlistment, based on their aptitude and physical exams. There are opportunities for "reclassment" (change to a different MOS, usually with a re-enlistment or if the Army determines there are too many Soldiers in their current MOS or have a serious shortage in another MOS,) but the majority of Soldiers remain in their chosen job until their contract is ended or they retire.