What are these "subtle differences" that separate ME from CFS? I know that some like to establish that ME is entirely different from CFS but what makes it so when they symptoms are the same syndrome found in both descriptions. This goes on to muddy waters more for the future of understanding and the treatments of all sufferers.
The ME/CFS umbrella term is a catch-all diagnostic criteria. It doesn't diagnose a single specific disease, but it just gives a mish-mash of fatiguing illnesses simply by ruling out other diseases. After a doctor has ruled out other fatiguing diseases, then you're left with a syndrome, CFS, which isn't a specific disease, but an umbrella term.
What many people want is a way to diagnose a specific neuroimmune disease, known as ME. One way of doing this is to insist that 'post exertional malaise', for example, is one of the symptoms, because it is almost unique to ME, whereas other fatiguing illnesses don't have this feature. The fukuda definition does not require 'post exertional malaise' as a symptom for a diagnosis. The Canadian definition does require it. There are also other symptoms which could be used to give an ME diagnosis, which are specific to ME, and there are also now biomarkers which could be used to distinguish ME from other fatiguing illnesses.
The reason I think it is so important to separate ME from other fatiguing illnesses is so that research and treatment can then be meaningful. I think it would also help to remove the stigma surrounding ME.
Hope that's helpful, and makes sense.