@dahukanna @garrett It doesn't need to be that complicated. There is one discipline: UX design. A startup makes its first hire a UX designer. As the team grows, they hire specialists, e.g. a user researcher, a visual designer, and a prototyper to fill out the team. As these skills aren't locked behind a single type, it's possible for one person to do many of them.
This is just like the term programmer which can literally do everything, but as the team grows, branches out to e.g front/back end