sentences:  The total number of active-duty four-star generals in the Air Force is limited to a fixed percentage of the number of Air Force general officers serving at all ranks.Within the Air Force, the chief of staff (CSAF) and vice chief of staff (VCSAF) are four-star generals by statute.Other four-star generals occupy positions of designated importance; historically, these have included the commanders responsible for strategic bombers and nuclear missiles (SAC/STRATCOM); tactical air combat (TAC/ACC); air transport (MAC/TRANSCOM); North American aerospace defense (NORAD); the Air Force formations in Europe and the Pacific; and other training, readiness, and materiel organizations.