In the Autism communities, the terms “high functioning” and “low functioning” are used quite a lot. However, when asked, the communities are unable to agree upon defining criteria for each. This has led me to look for a clinical description for each, specifically for Autism. However, I was only able to find the Global Assessment of Functioning (GAF), which is not Autism specific but can be applied to all diagnosis in the DSM-IV-TR. 

The GAF does not use the terms “high” or “low” functioning, but uses a numeric scale from 0 to 100, based on a person's overall and cumulative rating in social, occupational, academic and psychological functioning. While it does include communication, it is not completely based on one's ability to speak. 

I recall that some of the community definitions used speaking and some used IQ as the defining criteria for functioning. However, there seems to be problems with that. For one, there are critics of IQ test results who say that IQ tests are only for measuring learning, not for assessing ability. Researchers have been critical of IQ tests towards Autistics since a lot of tests are language and cultural reference specific, and results are inaccurate. As for using speaking as a defining criteria, it has been noted that the ability to speak does not indicate ability to communicate and articulate, nor does it accurately represent abilities in other functioning areas.