Hi Laura, yes your lip acts as a cushion between your lower teeth and the reed. I tried for a close-up on this but it's quite hard to show a full view on this.
When I point to my lip I try and explain that it's not the top part of it that is used, meaning the outer lip where a woman applies lipstick. It's down lower between 1/4 to 1/2 inch below from the top of your lip.
Another close way to describe it is pretend you're sucking your thumb like a baby does. It's not pulling your lip over your teeth. If you look at my close-up you can still see the top part of the lip as you would by doing the thumb sucking exercise.
It's fairly relaxed but still a bit firm for control… in other words, if you relax everything you wouldn't be left with any embouchure support.
A beginner will get a sore there because that part of your lip is delicate and it takes a while to develop it.. like I said, this is natural though and don't think that you feel a sore it's because you're doing it wrong.
