Billets for AD personnel are set at a higher level; HRC and the Surgeon General's office, at least in the Army. Hospital CO don't have that much power in terms of creating new active duty spots, especially for physicians. For enlisted folks, they can rotate people to the hospital who are assigned to various clinics or units within the post that the hospital is in. However, each Brigade or unit has it's CO who has equal say in allowing enlisted folks to help out at the hospital taking vitals or other corpsman type activity. Many allow this because it gives their corpsmen clinical experience that they can bring back with them to the unit to train other medics. The commanding General officer has the most influence in getting a new active duty position. I know this because I PCSed to a location that previously didn't have a position for my speciality because the commanding general at the base made a stink about creating a new slot there and HRC and the surgeon general's office signed off on it.