I don't know if I would call what I do temporary remissions, or something else entirely.
Spontaneous Remissions are usually remissions in cancer patients mentioned in the literature, where the patients actually and opposite to spontaneous, often applied lots of diet- and lifestyle changes very persistently and for long periods of time. Since no randomized evidence with lifestyles, therefore 'spontaneous'. Remissions with cancer usually should last 5 years.
I simply shortcut this lack of clear and logical definition, by clearly indicating: remission of 'symptoms' only.
In my case, from PEM's, no more lasting longer than the same day. Basically as in youth working up to 10-12 hours on construction sides (of course, with much fewer hours now; 3-4 only). Totally exhausted and in pain. But cleared the next days. This remission of a particular debilitating symptom, lasting now for 6 years.
Or in the case of remission of symptoms of COPD was a very persistent chronic bronchitis for the whole year of 2012, diagnosed only afterward as COPD stage 1, with symptoms already gone: 12 years remission from symptoms of COPD.
In the case of my first diagnosis of a chronic disease of PAD 16 years ago, the remission proper from its walking disability lasted exactly 5 years. Due to corona measures then, I couldn't engage in some life-style changes anymore (like 6-7 weeks of extensive sun-bathing in the tropic, swimming etc. during the deepest winter), therefore some, but better bearable leg-pains returned since. With worsening as ever slight, I would speak of remission in the past only. With worsening of main symptoms again, which at least obvious to me, reversed remission.
So I think with not so much definition clarity in the term 'remission', one certainly could also use that term for
temporary remission of symptoms, with temporary improvements. Shorthand, calling it remission of the whole ME/CFS disease, seems a little shortsighted to me.
Lung or heard expertise wouldn't be able to cancel my diagnosis, they still see the clear signs of it with their sophisticated apparatus - unless I told them, I'm free of its obvious symptoms. Walking disability or shortness of breath. With ME/CFS no obvious apparatus to detect, the situation is of course the more tire.