Thanks Fligh, I can see now how it m,akes sense. I have a version of Nozzy’s rig - I thought he’d layered the bones and hidden the layer but now you mention it, I think they are hidden bones. Also mkae sense.
Womball: RVKs are for storing a collection of deformations of a mesh. So in short, you move save a base mesh then move a part (mouth, eyebrow, whatever) and store that. Then you go back to the base mesh then move a different part then store that. Each change is stored as a key and the moved vertices are stored “RELATIVE” to the base mesh (hence "Relative Vertex Keys).
Each key can then be placed on a timeline and given a weight via a slider. So, if you have a key with mouth open, you can set the slider half way and the mouth will open half way and a key will be set at that point. Move the timeline forward, save a key with mouth open set to zero and the mouth will return to the base mesh position. You can set anumber of keys at one point and each deformation will occur together. So you can open mouth and blink at the same time using multiple keys. If two keys compete for the same vertices, Blender averages out the deformations at those points.
There is a pretty good RVK tutorial in the blender docs plus a few good ones right here on elysiun.