Low function or high function people?

Right now, I am waiting for client's daughter to call me back to confirm or cancel the appointment. Whole mess was started when the daughter got involved and called me up demanding for details which I provided so she is checking them out and see if it's worth it or not. It seems like the client depends on the daughter a lot. 

While I was surfing the sites minutes ago, the subjects of low-function vs. high-function came to my mind. I have heard the term, "low-function person" frequently and was always associated that term with those people who inadequate educations or ability to have conversations intelligently. 

But it that always the case?

Because I have met some people who have college degrees that I utterly couldn't believe that they even have high school diploma let alone college diploma! But it is not my position to judge. 

So... What really make some people "low function or high function?"