Always use the correct term to refer to "ME".
Myalgic Encephalomyeltis/ Post Viral fatigue Syndrome (ME/PVFS) is formally classified as a neurological disorder in the International Classification of Diseases (ICD10:G 93.3; WHO 1992).
“CFS”, according to the WHO is only acknowledged as a colloquial, not an absolute term to represent ME. The ICD separately classifies fatigue syndromes as a behavioural (psychiatric) disorder (ICD 10:F 48).
What the PACE Trial refers to “CFS/ME” is a condition of “medically unexplained” fatigue that is perpetuated by inappropriate illness beliefs, pervasive inactivity, current membership of a self-help group and being in receipt of disability benefits (PACE Trial Identifier, section 3.9).
“ME/CFS” is an umbrella term, subject to many different interpretations, incorporating a wide range of meaning and poorly identified conditions.