Welcome to FT doreen_in_sf and well done on searching the obvious places for answer.

CPRich has already answered for RDM and BIS. Each airline may have its own meaning for booking classes, so best to ask on the specific airline's forum.

I think on UA, "C" means paid (as opposed to award) full fare business class and "D" is paid discounted business class (note there are other discounted business class codes too).