HFA is not an official diagnostic term. There is only Aspergers Syndrome and Autistic Disorder in the DSM-IV. HFA does not rate a mention. Intelligence has no part of where someone falls on the spectrum. IQ tests are just a tool, something that helps identify strengths and deficiencies in different areas. If you do poorly in the Verbal component, it is apparently a strong indicator that you are autistic. It is by no means a certain indicator, however. Diagnosis is not a precise science.
Some people use the terms interchangeably, but there is a lot of debate as to whether they are the same thing, and how they differ. In the end, a lot of it is just a personal decision. I feel there is a difference, but a large part of it also seems to be due to personality rather than neurological wiring, but how much is personality the result of neurologics anyway? People will insist that, in the end, it is all autism, but that shouldn't stop us from categorizing; having the one umbrella term of "autism" would be trouble, imho.
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"Hitting bottom isn't a weekend retreat, it isn't a goddamned seminar. Stop trying to control everything and just let go!"