I think the theory is something like certain people with certain genetic disposition develop CFS after certain viral/physical/emotional stress. Sort of like not all people developing PTSD after a stress. My favorite is a brain/ANS injury from immune distress caused by a viral infection or extreme physical activity. Mind you, it's only one of my idle musings, 'cos I have nothing else better to do. Just in case anyone mistake that for a theory with any scientific basis. Going back to the topic, I have no idea what Kerr is trying to accomplish with subtypes. I suspect it all goes back to the waste basket theory of CFS. I'd consider CFS as one disease with variable serverity till proven otherwise.