Looking at the dictionary definitions, exertion isn't really to do with expending energy or being active.
It means making an effort, or applying a force (which potentially requires no energy if there is no movement even if in biological systems it usually does).
It certainly isn't a stimulus or stressor, as far as I can see.
It involves doing which is a difficult concept because it is tied up with agency and we don't understand agency well. Until we do, I suspect exertion is not a term that can be used in a scientific theory beyond the reference to some subjective sense that someone has made an effort.
I think there is quite a serious problem here because PEM is central to arguments about the causation of MECFS but the exertion bit is highly ambiguous and very open to misinterpretation. Lots of people seem to assume that it implies using energy or being active. If so, it is likely the wrong word.