Formerly, the grade of sergeant was obtained after a time as a senior airman and successful completion of the Air Force NCO School. The Air Force and Space Force are the only ones of the six branches of the United States military where NCO status is now only achieved at the grade of E-5. Above the pay grade of E-4 (E-5 through E-9) all ranks fall into the category of NCO and are further subdivided into NCOs (E-5 and E-6) and senior NCOs (E-7 through E-9) the term junior NCO is sometimes used to refer to staff sergeants and technical sergeants (E-5 and E-6). While all Air Force military personnel are referred to as airmen, it can specifically refer to the pay grades of E-1 through E-4, which are below the level of non-commissioned officers (NCOs). First sergeants revert to their permanent rank within their paygrade upon leaving assignment.