The idea that individuals with autism can be evaluated in terms of a single dimension describing how well they "function" is both popular within the medical community and viewed as problematic by the autism community.
The most common functioning labels are "low-functioning" and "high-functioning", usually with "autistic" or "autism" appended ("has low-functioning autism" or "is a high-functioning autistic person"). The now-deleted diagnosis of "Asperger's syndrome" was essentially equivalent to what is now commonly described as "high-functioning autism".
These "functioning" labels are less than ideal, for reasons that include:
- Ability to function varies in lots of ways and doesn't really fit well into just two categories.
- Autism varies in a lot of different ways, so someone's ability to function can vary a lot depending on contexts such as what it is they're trying to do, how much support they have, and generally how they're feeling/coping at any given moment.
- It can be harmful to "high-functioning" people, as others may assume they don't need support: "but you're high functioning, of course you can do it!"
- It can be harmful to "low-functioning" people, as others may assume they're incapable of things they can do: "oh, no, of course they can't manage to do this for themself, they're low functioning!"
In general, these evaluations tend to be made from an external perspective (i.e. by others, not by the autist), and also tend to focus on capitalist ideas of merit such as "productivity" rather than being oriented toward the individual's well-being and need for self-expression and agency.
Some better language for talking about this is "cultural compatibility". Someone who cannot speak or communicate symbolically but who is good with animals and simple repetitive tasks might be considered very "low functioning" in our culture, which depends heavily on language, but might do well in (for example) an agrarian culture, where watching over animals or carrying out simple routine tasks was highly valued. In other words, that person has "low cultural compatibility" with our high-verbality society, but might have "high cultural compatibility" with a more agrarian society.