What does academic with regard to member type in UE mean?
To me, it is someone who is a member of a university or similar institution, probably either on the teaching or the research teams, possibly both.
It's probably a distinction that's included as a possibility because colloquial English often implicitly excludes university professors from the terms "teacher" and "doctor" though we're actually both. You wouldn't believe how often North Americans answer "Oh, I thought you were, like, a real doctor or something, heh heh heh" when they hear we have no medical specialty. And to think, "M.D.s" only really hold undergraduate degrees!
Yeah, MDs and JDs are conferred a doctorate title upon gaining their first degree without having any research done, any publication contributed to to any one journal. These are not real (research) doctorates; MD/PhD and JD/PhD are.
One more question to make things straight: An MD/PhD, for example, who is apparently not trained in the field of the English language, is (s)he an academic too in the sense UE specifies?
Firstly, if you don't speak good English, you should tick "learner" even if you have a PhD. That is, if you are here primarily to learn rather than to teach, you should not tick 'academic'.
If you are not an English teacher, but you are here primarily to teach; and you have an academic interest in English language or linguistics, you should tick 'academic'.
If you have a range of interests, and teaching English is only one of them; and you have other academic interests, you should tick academic. This is why I ticked it. I think "English Teacher" is better suited for full time teachers or those who've had significant professional experience in ESL teaching.
Maybe whoever came up with the choices could explain their motivation?
Well, I have academic interests, but English is not one of them because my English is far from that level I can safely call academic. I am primarily here to learn by teaching. I think interactive ("students" answer back ) teaching is the best way of learning. I do not think I would fit any of your categories. Therefore, I remain svartnik.