Dani Alexis, aka the Autistic Academic, writes a to-the-point essay on the problematic nature of "functioning" labels with regard to autism spectrum disorders (ASD). Words are powerful & these particular adjectives have a tendency to betray the humanity of those they are meant to describe.
"I have heard 'high-functioning' used to mean many things, from 'has an IQ several standard deviations among the mean' to 'has a job' to 'talks.'
But I have never, not once, heard 'low-functioning' to mean anything but 'hopeless tragedy.' And I have never heard either of these labels deployed to mean anything but 'still not quite, you know…one of us.'