To the best of my knowledge, values C and D are only used in the net players checklist...
Net players is the only checklist I ever use, so others might use them, but I don't know of them - also I can only account for DBC, DBP might be different if that's what you're using...
I'll check out each of DBCs checklists and see what they do (but limb checklist in particular)...
Edit: To do with limbs - this is what I got
checklist value a - The number of the limb
checklist value b - -1
checklist value c - -1
checklist value d - -1
so, I can't help much either...
...maybe one day I'll finish a project