Autism, as is so often stated, is a spectrum. Often, however, this notion is used to support a false binary. There is a common misconception that if you are autistic you belong in one of two categories: “high functioning” or “low functioning”. If you're considered “high functioning” you likely speak, attend mainstream school, and mask (i.e. hide) your traits well enough to function in a social setting. Typically, “low functioning” is used in reference to non-speaking autistic people, particularly those with a co-occurring learning or intellectual disability. Human beings tend to slot things into neat little categories with neat little labels. The problem with functioning labels, is that they just don't cut it; they are both redundant and offensive. So often “high functioning” is used as a synonym for “savant” or a euphemism for “not really autistic” — either way, it is often used to justify ignoring a person's support requirements.
Conversely, “low functioning” is used to peddle the notion that speaking is the superior mode of communication, and that autistic people with a particular set of consistently high support needs are incapable of autonomy. The idea of autism as a spectrum is supposed to illustrate that being autistic affects no two people in the same way; our difficulties can vary depending on time, place, or context. To assign such rigid labels entirely misrepresents the experiences of autistic people all over the spectrum, a great illustration of this can be found at Art Of Autism.
I'm still on the waiting list for a formal diagnosis, but from the research I've done myself, the results have almost all declared me to be 'high functioning'. But like the author of this article, there are days when I don't feel remotely like I'm functioning highly.
I suspect that the desire to place autistic people into various brackets is driven by the motivation of those doing the grading, which in turn is driven by the perception of autism as meaning either 'genius' or 'retarded'. The former may be supported, though not necessarily tolerated, while the latter will most likely have to fight for any support at all.