I would differentiate susceptibility to fatigue, and actual fatigue (the later being present even without exertion).
Susceptibility to fatigue means my muscles feel fine, but when I start to use my muscles by engaging in a physical task, then I notice I rapidly lose muscle strength, and so my muscles start to feel physically tired.
As opposed to feeling tired in the body and muscles even without doing any exertion. Some days I can wake up and my whole body feels weak and tired, even without doing anything, and without doing anything on the previous days (not PEM in other words).
You could make the same distinction for mental fatigue, I guess. You can just be tired mentally from the moment you get up, even without engaging in any mentally exerting tasks. So that's a mental fatigue present without exertion.
But it is also common in ME/CFS for people to experience rapid mental fatiguability, where they engage in a mental task, and find that task much more mentally exhausting than a healthy person would.
But anyway, the
ICC talks about muscular and mental fatiguability (under the PENE section), but it does not mention fatigue.
Whereas the
CCC mentions muscular and mental fatiguability (under the PEM section), but also refers to fatigue as well, in its own section: