JMRI's Decoder Pro already supports the CT Elektronix DCX decoders, and loads the appropriate menus in the programmer when you attach the decoder to the SPROG.
CV8 identifies the manufacturer of the DCC decoder (not the manufacturer of the locomotive.) It is NOT TO BE CHANGED. The other CVs are used to customize various functions, and all come pre-set with default values. For instance:
the locomotive number; most DCC decoders come pre-set to "3". The locomotive buyer will change this to their own number after purchase.
forward and reverse (good to know in case you install the decoder "backwards)
headlights (the yellow, blue and white output wires) are set to be directional; the owner can change this
the motor starting voltage and maximum voltage are set by the decoder manufacturer, but these can be changed (the PDF button is a tutorial by Brian Vianco which explains how to set CVs 2, 5 & 6 for the PowerMAX)
speed curves are set, but these can be customized by the owner
momentum and braking are set at a "medium" value, but can also be customized
other features such as ditch lights, etc.
sound decoders have LOTS of features you can customize, from selecting specific whistles and bells, to reverberation rates, etc.