In the eastern Sierra "D" is the "detail officer." Their duties included working with the media, giving school presentations, and inspecting school buses.Their might be some others duties, but I've forgotten since retiring from the USFS where I interacted frequently with the CHP. The Bishop Area Office and the Bridgeport Area Office each have a D unit, i.e. 71D1 and 72D1. I've never heard dispatch or the officers pronounce "David," they always use "D."
There used to be, and may still be, a "mechanic" at each area office that changed oil, rotated tires, mounted and balanced tires and the like. They had the unit identifier of "71D2."
When I'm visiting southern California I hear all sorts of letters in the unit ID's and quite often don't understand what I'm hearing. Up here, Robert units are not rovers, but reserves. Not reserves like sheriff's department have but regular officers brought in from out of the area to handle special workloads such as special events, such as Bishop's famous "Mule Days."
Most of the information I've presented here is not only as a result of working with the CHP but also from my neighbor of 7 years, who retired as an officer during the time I lived next door to him.