What is the primary purpose of acceptance criteria in a user story?

Acceptance criteria primarily serve to clearly define the conditions under which a user story is considered complete. This set of predefined requirements provides a checklist for the development team to determine whether the functionality meets the necessary standards before it can be marked as done. By establishing specific criteria, it ensures that both the development team and stakeholders have a mutual understanding of what the deliverable will entail and what is required to satisfy the story's goals. As a result, acceptance criteria help maintain accountability and transparency throughout the development process, making it easier to manage expectations and deliver quality outcomes.

The other options focus on different aspects of the software development process. While outlining requirements for user testing, defining the expected user experience, and establishing project timelines are all important activities in the framework of agile development, they do not capture the primary function of acceptance criteria, which is specifically concerned with marking the completion of a user story.