Yes, that is what boundaries do. They keep people from crossing the line. It's usually a fairly strict line that is set up between person A and person B. Person A might have a different boundary with Person C.

But they are rules.

And there's nothing wrong with that. No one has to be fluid with what makes them comfortable or uncomfortable. They can indeed have a very absolute rule.

The difference between a T and most other people - is that the T will make those boundaries explicit (at least most of them) so that the client is aware of it. And the T is likely going to be a bit more forgiving when those boundaries get crossed - to an extent. And when the very clear boundary has been crossed, the T can point that out, and can point out that the client was aware of that boundary already, and then they can discuss it and how it affects the client. If the client continues to ignore that boundary, then the T will possibly decide to terminate because it is not healthy for the T anymore.

When the boundaries are blurry and unfocused, then the client is unable to see what's acceptable to the T or not, and thus is either going to be excessively worried about crossing those unknown boundaries..... or they'll go the other way and decide that they can have total control over what they do to the T and feign innocence when the T expresses displeasure at the boundary being crossed.

Boundaries set up the boundary for what's ok and what's not. It's like having a big indoor ballpit - the glass barrier is the boundary. Real-life on the outside, therapy on the inside. What the T puts into that ball pit is up to them, but the client knows very clearly what's there and what is alright and acceptable. It's not refusing to let people in. It's not keeping someone out. It's showing what is and is not acceptable to Person A. It's not making Person B guess. It's being as transparent as possible to help avoid confused and undue upset.