The functioning labels are often used to refer to the "severity" of someone being autistic.
When you call someone "low functioning," you usually think of someone who is unable to communicate, participate socially, take care of activities of daily living independently, etc.
When you hear of someone call someone "moderate functioning," they may have some communication, may be independent with some activities of daily living but not all, among other things.
When you hear someone is high functioning, you think of someone who is unnoticably autistic. They communicate well but they are socially awkward. Some may not believe they are autistic.
Why do we have functioning labels? It is a way of describing how disabled someone is by a certain condition. Humans have a need to put labels on everything.
What do functioning labels actually describe? How other people experience us being disabled (autistic or otherwise), not how we experience being disabled.
How is this harmful? I can use my experience for example. I am what is considered to be high functioning autistic but I hate using functioning labels. I was diagnosed as an adult after my son was diagnosed. I went into a doctor's office and they wanted a list of all the diagnosis that I have. I list them as autistic, polycystic ovarian syndrome, asthma, etc. The nurse said "you don't look autistic" and of course the smart ass that I am, I had a reply. "What does an autistic look like?" Then she answered "You must be high functioning." She did not know it takes all my energy to go out in the community. I cannot handle phone calls well. I have sensory issues that can be debilitating and other issues. It felt like she minimize what I experience on a daily basis. It was meant to be a compliment but its really not. Its like a punch to the gut.
Take someone who is "low functioning." I heard this a lot as a kid since I did not speak until I was five years old. No one knew I was autistic until i was 32 but I was in intensive speech therapy. My mother, who is also a narcissist, was describing me as low functioning all the time. I was Deaf at the time but I could read lips well. Something I taught myself because she never bothered to teach me much. I could express by using American Sign Language but she never bothered to learn. People would see me visibly upset a lot of the time because of this. I was a child outside of my difficulties. There was more to me than my communication difficulties. When I "heard" low functioning, it hurt my self esteem. Everyone else was better than me. I was a burden. I felt low. It took a long time to get my self esteem back.
This is why autistics say functioning labels are harmful. Do not minimize struggles for someone who masks well and do not underestimate people who need more support. We are just autistic. Throw the functioning labels where they belong. Down the toilet.