If you are a line holder in the Washington DC base (DCA), 737 and 320 pilots have trips that originate at Reagan National (KDCA), but it is a smaller percentage than Dulles (KIAD) or Baltimore-Washington (KBWI). You can bid the DCA trips as a higher priority if you desire. If you are on reserve, you are expected to originate at Dulles, but do occasionally get called to go out of BWI or DCA. There is no ‘IAD’ domicile at United, it’s referred to as ‘DCA’ despite the vast majority of trips operating at Dulles. Clear?