A boundary describes what you will participate in
A boundary clarifies your own participation and limits. Saying that you will pause a conversation if shouting begins explains what you will do. It differs from demanding control over another person's feelings or choices. The boundary concerns your action, even when you cannot control theirs.
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Keep the limit within your control
A difficult conversation becomes louder. You say that you are willing to continue discussing the issue, but will step away if the shouting continues.
That statement describes the conditions of your participation and the action you can take. It does not require the other person to stop feeling angry.
A clear boundary is easier to understand when it is specific and connected to the situation. Vague threats leave people uncertain about what the limit means or when it applies.
Following through also matters. If you pause the discussion, an agreed opportunity to return may help where continued conversation is appropriate. The limit can protect how a conversation happens without deciding its outcome in advance.