There are very strict medical definitions. If you are "High Functioning" with Asperger's and you are not functioning well, you are still not low functioing.
LFA is a set of behaviours that are observable. I hate to say Rain Man, but that is what they are looking for, fair or not.
HFA and not funtionging would indicate co-morbid states, like depression, etc.
A LFA autisic may function way better than a HFA depresseed Aspie. In fact, I used to work in a field where there were HFA nad LFA and I was like, "OMG, the LFA are doing WAYYYY better than I am!"
They had tons of support, no depression, were in their own little worlds, allowed to stim and engage in their obsessions freely while I was forced to do all this SH**** like smile, pretend not to be thinking of my special interest, not stimming etc.
So you can be LFA and functioning very well, but just not as NT's function. That is the key. They (NT's) call it LFA, but to me, it's BOOOM. Good on ya!
It can be stimming a lot, repreating words, "odd walking" not making eye contact, all the things that frighten NT's soooooo much.
HFA do not frighten NT's, we seem to p*** them off. We smile (YAY) but at the wrong time. We talk (YAY) but don't make eye contact. We are close enough to NT that they are like, "WHAT IS YOUR PROBLEM?" whereas LFA is indisputably NOT NT.
FOr me, Not talking was one of the keys. WHen I was young I did not in public and if I had kept THAT ONE TRAIT, I would have been considered LFA. I had way enough other ways.
But I was forced to talk and that complicated the whole thing. Then I had to learn to smile, and had to learnt to socailize, then it was downhill from there.
For many years, my New Years Resolution is to stop talking but I can't now. Family dynamics were set early, around my talking.
My only hope is when I get dementia, I can stop talking again. That happened to my ASpie unlce who HATED TO TALK, too. Rest his dear soul.