It's an interesting term.
Great question.
Within Bipolarchic's thread, I did reply; however, not because I can work or do much at all. I have serious physical disabilities and serious c-PTSD and mood-related issues. The point I had tried to make is: Doctors/others often do not initially take my requests for help very seriously because of my ability to articulate, etc. This is mistaken for "high functioning," esp if they do not read my chart/history.
Most of us are rated by our pdocs/therapists on a "Global Assessment of Functioning" for the past year.
Here's an explanation of scoring for adults and children, if interested:
https://www.omh.ny.gov/omhweb/childs...unctioning.pdf
Since I've been repeatedly exposed to disability reviews which have included the use of this scale, I tend to think in terms of this scale.

WC