Wow, what great responses and useful information! Thank you all so much! I really appreciate the effort and thought that you've put into your explanations!

Is certainly does sound like there are lots of different kinds of fatigue, and even within those categories, it varies somewhat from one person to another. Just like MS in general, hunh? So I'm getting from this that what I need to emphasize to the doctor is muscle fatigue and spasticity* after minimal exertion.

*I initially mispelled that word, and found it hysterically ironic.