The key word is "functioning". I function quite well. But I also have a 99th percentile IQ so I have been able to construct a large number of compensatory mechanisms. But I expend a great deal of energy executing these mechanisms and it leaves very little room for "living in my own space" for lack of a good description. Functional levels are often assumed to be equivalent with severity levels. While there is definitely a correlation, this is a simplistic view. My social INTUITION is SEVERELY affected. I at times am fully blind to the social environment and communication that is happening around me. I'm just able to figure things out on the fly well enough that I get by.
So to answer your question. Low functioning, in general, is a person that needs a great deal of assistance in daily living. High functioning is a person that, in general, is able to get by with little to no assistance.
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When God made me He didn't use a mold. I'm FREEHAND baby!
The road to my hell is paved with your good intentions.