Rather than thinking of a user story as a requirement, I find it more useful to think of each story as a pointer to a requirement. Every story is a placeholder for a future conversation where stakeholders convey the needed details. A user story is too vague to be a requirement; it's a pointer to a requirement.
The product owner writes acceptance criteria for user stories. Acceptance criteria, higher-level than test cases, are the conditions of satisfaction that if unmet result in product backlog item rejection.
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