The language of boundaries is now widespread and frequently used to describe something else entirely. A boundary specifies what you will do, and most of what is called boundary-setting is instead an instruction issued to another person.
The distinction is who the statement governs
A demand tells someone else what to do and depends entirely on their compliance for its effect, which places the outcome outside your control.
A boundary states what you will do in a given situation, and it can be carried out regardless of whether the other person agrees with it or not.
The difference is not a matter of tone. It determines whether the arrangement holds when the other person is uncooperative, which is precisely when it is needed.
Enforcement is the whole of it
A boundary that is stated and then not acted on teaches the opposite of what was intended, because the observable pattern is that the stated consequence does not follow.
This is why boundaries requiring dramatic or costly action tend to fail: the person setting them cannot follow through and so does not.
A modest boundary that is reliably acted upon does more than an ambitious one that is announced and abandoned.
They do not require the other party to agree
People commonly delay setting a boundary until they can make a case the other person will accept, which makes the arrangement dependent on winning an argument.
Because a boundary concerns your own conduct, it does not need to be justified to the person affected by it, and continuing to argue for it usually weakens it.
Disagreement is a normal and expected response rather than evidence that the boundary was unreasonable or badly explained.
The purpose is sustainability, not distance
Boundaries are frequently presented as a way of protecting oneself from others, which frames relationships primarily as sources of threat.
Their more useful function is making a relationship sustainable at a level of involvement that can actually be maintained over time.
Without them people tend to overcommit and then withdraw entirely, which is more damaging to the relationship than a consistent and smaller commitment.
Where the framing does not apply
The language assumes roughly equal standing and the practical ability to act on a stated position, which does not describe every situation people are in.
Where there is dependence, control, or a real risk of harm, advice to simply set better boundaries misdescribes the problem and can place responsibility on the wrong person.
Those situations call for specialist support rather than communication technique, and that distinction is worth drawing clearly rather than assuming the framework fits everywhere.