High/low-functioning labels are meaningless. You can see examples on this thread; people don't know exactly what they mean--and it's not because they're ill-informed, either. It's because every doctor you meet will have a different definition of them. There is no official definition for those terms, and if you use them, you have to clarify what you mean by them every time.
You can go by GAF, which is a 1-100 scale that shows exactly how you are coping right now in your daily life. On the GAF scale, I range anywhere from the 20s to the 60s. Right now I am in my usual spot in the mid-50s, which is common for disabled people in general.
You can talk about how good someone is at communicating--whether they can communicate sometimes, always, or communicate some things but not other things--like being able to answer questions but not spontaneously explain something that they are thinking about. On that scale, I am toward the more skilled end, able to speak almost all of the time, read and write all of the time unless in complete shutdown, start conversations on my own, and make up my own sentences fast enough to participate in a normal-speed conversation with up to two people at once.
You can talk about how good someone is at taking care of themselves, all the way from things like remembering to eat, to doing your own laundry, to earning money for yourself. That's not a one-dimensional scale, either--some people who can work might still be reminded to eat, for example. Me--well, I can't work yet; but I can't go to school. I can do my own self-care and don't need an aide, but it took me until my late teens to learn, and it still takes more time than it does for most people. I can clean my own apartment. I'm still routine-dependent for all of it, but the routines work, so why complain?
Those things don't really go together. There are people who can take care of themselves but not talk; people who have a low GAF score who are employed. Each case is different. Most people on the spectrum could probably be called "high-functioning" by some criteria and "low-functioning" by others. And it's even worse that lots of people think that "HFA" and "LFA" are official diagnostic labels that can be used to predict things about a person. They can't--but that doesn't stop people from hearing that you are one or the other and making assumptions that often aren't true.
Functioning levels, are meaningless. There's nothing you can say with functioning levels that you don't have to clarify afterward to make sure you're not misunderstood.