I think it really depends on both the scenario and the person; but generally speaking, healthy boundaries are when you know what makes you uncomfortable.
for example, i don't allow physical touch without my consent - sounds obvious, but i mean even touches like tapping me on the shoulder and such, since these overstim me easily and make me irritated.
Maybe somebody prefers to avoid specific subjects when talking. Maybe somebody says "no" to an offer and the others try to keep pushing them into doing the thing they disagreed to do - these are all personal boundaries.
it's important to let the others around you know what your limits are. And if somebody corpses one of them on accident, without malicious intent, it's good to just let them know that it upset you and explain why.