First response nails it. It's not a spectrum of mild to severe, it's more like a colour spectrum.
Much like how each colour is made up of a certain amount of red, green, and blue, each autistic person's experience is made up of a constellation of traits. In fact, in order to be diagnosed you must be significantly impaired by your symptoms.
I am highly verbal, for instance, and I can execute a basic routine to get myself through life- keeping myself fed, rested, and clean. I wasn't diagnosed until my teens, but with the help of my family I made my way through school, undergraduate studies, and a master's degree. Now I have contract for ad-hoc work in a field I can cope with. I have three close friends whom I love dearly, and many friendly acquaintances I'd love to get to know better. I travel when I get the chance, mostly around Western Europe, and I enjoy travelling by myself.
I also need help making and maintaining such routines when faced with unexpected changes to my plans, sensory overwhelm, illness, and more. I already need a lot of help maintaining my routine but if any of those happen, I will need even need support to make sure I don't spiral. I need multiple daily medications to uphold even those basic standards, the prescriptions and ordering of which are managed by my family. I learned a long time ago I need help keeping a space clean and tidy, otherwise I cannot maintain a healthy environment. I have a very limited and beige diet, which I struggle to break free from. I'm also prone to shutdowns, during which I can't speak aloud, and have during some times in my life been equally likely to have self-injurious meltdowns. I have my mother to help me with all of those, and I don't know what I'll do when she's gone. It terrifies me.
All in all, I know I'm considered "mild" in many people's understanding, but I don't think the label fits me very well. I have some mild and some more severe symptoms, and times in which those are made better or worse by my environment, overall health, and stress levels. Almost nobody sees what I described in that second paragraph on my traits, symptoms and experiences, but that doesn't mean I don't experience such struggles. I always have, and I always will.
I've learned through many discussions over the years that any "subcategories" proposed to delineate between different experiences of autism are unlikely to ever actually accurately classify me, nor the other autistic people I know. I think there needs to be better linguistic descriptors of symptoms and support needs so that nobody slips through the net, but "mild" isn't a word I think can be used to describe autism.
If you're autistic, it is something that filters through your entire life. It is an inherent part of you, and nothing about that impact is mild no matter how it may appear to others.