A boundary gets confused with a demand more often than it should. A demand asks your partner to change their behavior. A boundary states what you'll do if a certain behavior continues. That difference sounds small, but it's the difference between a boundary that holds up under pressure and one that turns into a recurring argument.
A boundary is about your own behavior, not your partner's
"Don't raise your voice at me" is a request you can't actually control the outcome of. "If the conversation gets heated, I'm going to step away and come back to it later" is something entirely within your control. The second version is a real boundary. The first is a wish, however reasonable, about someone else's behavior.
Boundaries aren't ultimatums
An ultimatum is delivered as a threat, usually mid-conflict, and tends to escalate things further. A boundary is calmer, ideally set outside the heat of an argument, and stated as information rather than a warning shot. The tone it's delivered in changes how it's received almost as much as the content does.
Vague boundaries don't hold up under pressure
"I need more space sometimes" is hard to act on, for either of you. "I need thirty minutes alone after work before we talk about anything stressful" is specific enough that your partner actually knows what to expect, and specific enough that you'll notice if you're not holding it yourself.
Expect testing, especially at first
A new boundary in an established relationship often gets pushed on initially, not always on purpose. Old habits and old expectations don't disappear the moment you state something new. Consistency in the first few instances matters more than getting the wording perfect.
A boundary that's never enforced isn't a boundary
If you state a boundary and then don't follow through when it's crossed, it quietly becomes a suggestion instead. That doesn't mean every crossing needs a big reaction, it means the thing you said you'd do actually needs to happen, calmly and consistently, or the boundary stops meaning anything.
Healthy boundaries aren't about controlling your partner. They're about being clear, to both of you, about what you need and what you'll actually do to protect it.