It seems like there isn't any agreed upon definition of high functioning, but I consider it someone who can speak, dress himself, handle tasks, etc., to at least a basic degree. I say that Hoffman's character straddles the line because he can do these things, but it was hard to imagine him living on his own without any assistance. Like I find myself saying a lot on these boards, it's a very broad spectrum we're dealing with here, and fitting people into two or three or even fifteen convenient categories isn't possible.