Build a mapping table that tells us the virtual parent for each bone in the ref skeleton. For example if we have finger bones that are not in the ML Deformer inputs, we try to find the closest parent bone in the hierarchy. So for the fingers this might be the hand bone. The vertices touched by the fingers will then be added to the mask of the hand bone. The table works like this: "Table[FingerBoneIndex] == HandBoneIndex". When the bone is inside the bone input list of the ML Deformer, it will just return itself, and not its parent bone. Basically it just tells us to which bone's mask to write to for this specific bone.