That's too broad, you can negotiate things like salary independently. Other things you cannot, and it's often to prevent them from saying "Sally agreed to lower her vacation time to 2 weeks, is that ok with you too?"
I still didn't hear how it prevents you from switching employers.
> That's too broad, you can negotiate things like salary independently.
For example I worked with a school where the area management and the union set salaries and individual teachers could not negotiate on their own. I'd say that's pretty typical in unions. It's also the case in for example the NHS and British railways (topical at the moment.)
> I still didn't hear how it prevents you from switching employers.