The terminology for describing what happens on the outer surface of a steel part after nitriding or carbonitriding can be daunting, that's for sure.
The term "compound" refers to chemical compound and is used to distinguish the surface features from the diffusion layer. If it helps, think of wood. Stain penetrates wood (like a diffusion layer), but the deeper in from the surface, the lower the concentration. The compound layer is something chemically different, like paint. It is simply a foreign layer.
http://burlingtoneng.com/case_hardening.html
http://www.struers.com/resources/elements/12/2475/35art3.pdf
As far as assessing surface treatments using Rockwell, it can only be done with Rockwell superficial. I see engineers all the time trying to call out HRC on a case hardened part. It's like smashing a hard-boiled egg with a sledg hammer and thinking the required force is telling you something about the shell. So if you must use Rockwell, at least use something like the 15 N scale.
One of my pet peeves is drawings with case depths which call out something like "56 Rc min at a depth of 0.01" " This is an unverifiable spec! Always keep in mind that Rockwell does NOT measure hardness. It is a measurement of depth. Scratching something with a file IS a hardness measurement. So really if waht you want to know is how hard something is, especially if it is a thin layer, a file or scratch test makes more sense.