As with so many things aeronautical it came from the nautical past.... That's also where we get port, starboard, knots (as a speed indication), the color of running lights, etc.... souls refers to all persons aboard without making the distinction between officers, crew, pax, etc... I've been a medevac freak for many years and have a bunch of them programmed into my scanners... Kentucky is blessed with a bunch of good folks flying around in choppers saving lives... The Association of Air Medical Services has .pdf files showing the ten minute fly circles for each medevac service in the country... this is
KENTUCKY's file....