Question:
How do people determine what is "high" functioning versus "low" functioning, etc?

My Opinion:
I have read much discussion on this subject on several different forums, but I will give you my own personal opinion here. It is by no means the exclusive opinion tho.

For most people including the medical types who do the diagnosing, the difference between autism and Asperger is the timing of the acquisition of language. If your child developed language by about age 2-1/2, then he would receive an Asperger dx. If he developed language later than that, so therefore "delayed", he would receive an autism dx [or PDD, depending on other factors]. This would be true even if you had two identical children, with identical issues, stims, difficulties, etc, and one developed language at age 2 and one at age 3, the first would be Asperger and the second autistic.

So therefore, I believe that higher functioning autistic individuals include Asperger individuals [altho many people believe Asperger is not part of autistic spectrum, but I disagree and say that it is part of autism]. And my own personal definition of high functioning is a person who is able to take care of personal needs, communicate desires, if a child can be integrated into regular classroom with little or no supports, and can live independently with little or no help when adult.

Mid functioning are those people who can take care of personal needs with little or no help, communicate desires, but would have tremendous difficulty in regular classroom or living independently without support. Low functioning are those people who need assistance for all of these things.

Now having said that, I do know several people who by personal example, challenge my definitions. They have very much difficulty functioning "in society", just a trip to the supermarket creates major anxiety and stress, and some days they simply cannot do it. Some cannot manage their own finances etc. I still consider these people to be higher functioning, even if living at home with their parents as adults.

Adults who live in group homes or other similar settings are probably more mid functioning.

Rainman I see as mid to lower functioning. From what I remember of that movie, he can take care of his basic needs like feeding himself, dressing, using toilet, communicating to some extent with others, etc. But I think he needed assistance, and I cannot see him shopping or things like that. I do not see him living independently, but perhaps a group home setting [not institution like in the movie], so perhaps mid functioning. I know of at least one autistic adult with the mental age of 12 months, who still wears diapers and needs assistance to eat. This person is obviously low functioning.

So I don't see the definition as how many stims a person has, or how rigid the routine, because I know many people with quite a few stims, who created many rigid routines in their lives, but I would consider them high functioning because they can basically live independently, altho many of them are not employed because working is too difficult and overwhelming for them. I also know at least one person who is entirely unable to communicate orally, he must type or use pictures, he cannot speak. However I consider him high functioning because if he wanted to, he would be able to live independently, by writing and things, similar to a deaf person.

So my opinion is that the functioning level is based on ability to live and function independently, whether or not it includes autistic-like behaviors or looks "normal" or whatever.

Well I hope that helped you a little. If someone else has a different opinion, please do write it here, so we can hear another perspective.