In the autism community, we tend to label autistic people as being "high functioning" or "low functioning" in accordance with how well we think they function in society. You've already likely guessed one problem with this - where's "medium functioning"? - but the problems with this labeling system go much further than that.
The first problem is what does functioning mean, exactly? Does it mean the ability to communicate verbally? Or intelligence? What about people like Amanda Baggs, who are quite intelligent, yet does not speak verbally nor hold a job? Is it, then, holding a job? Well, not exactly because there are plenty of "high functioning" people who don't have jobs. You get the idea. Functioning consists of so many different variables, and to apply one label is to gloss over the natural variation from person to person.
The second problem is, it over-generalizes. A person's level of functioning varies minute by minute. Some people, for instance, can verbally communicate very fluently most of the time, but sometimes can't speak at all. And meltdowns and sensory overload mean that a person could be functioning well in society one minute, but the next not hardly at all.http://musingsofanaspie.com/2013/06/26/decoding-the-high-functioning-label/
Misconceptions are another problem. The biggest one is basically that a "high functioning" autistic person is somehow less of an autistic person than someone who is "low functioning". There are the stereotypes that come with that - we tend to think "high functioning" people need/don't need certain accommodations and the same with "low functioning" people instead of taking it case by case. Plus there's this whole idea that the words of "high functioning" people doesn't apply to "low functioning" people, which is generally a pretty arbitrary difference.
And then, of course, the fact of the labeling itself is an issue. We give autistic people these labels, they're not generally self-given.
The labels also don't stand up to clinical tests of validity and tend to be based simply on the impressions of others as to how well an autistic person can pretend to be neurotypical. Also, categorical labels aren't generally conducive to spectrums anyways, so there's another problem.
I could probably keep going, but I think I covered the biggest points. I think we say "high functioning" and "low functioning" because they're short, easy labels. Even I will use them at times, to describe how a person seems to the rest of the world. But I rarely do so and always use quotes to make the fact that it's a serious over-generalization clear. I suggest you do the same.