"Loitering on Scene" = hanging around and finishing up. It's pretty much what you see with any emergency vehicle that's stopped and working on something with the engine running - and the switch increases the idle to 1000-1200 or so (as mentioned) to keep the alternator putting out current, since it really does very little at idle speed. It can also keep other accessories off an running - say, a hydraulic power unit from the PS pump or somesuch.
It's really too bad it's way out there, I've been wanting a PD-spec for a bit now, since it's got a couple things that are really handy for me and that I just don't feel like duplicating (read: "My wife doesn't mind my XJ collection!")
Gawd, California sucks...
5-90