According to the Agile Practice Guide and the PMBOK® Guide, specifically during Backlog Refinement (Grooming), user stories must be refined until they are " Ready " for the team to pull into an iteration.

Definition of Ready (DoR): For a team to understand the work needed to complete a story, the story must contain Detailed Acceptance Criteria. These criteria define the boundaries of the user story and provide a specific checklist that must be met for the story to be considered " Done. "

Determining Effort: Acceptance criteria are essential for the team to estimate the effort (Story Points) required. Without these details, the team cannot know if a story is a simple task or a complex endeavor. They act as the " test cases " that verify the functional requirements from the user ' s perspective.

Eliminating Ambiguity: During grooming, the team discusses the story with the Product Owner to clarify " what " success looks like. These clarifications are documented as acceptance criteria, which directly inform the technical tasks the team will perform during the iteration.

Analysis of other options:

Team velocity (Option A): Velocity is a metric used to plan how many stories the team can take on in an iteration, but it does not describe the specific work needed to complete an individual story.

Related epics (Option B): Knowing the parent Epic provides context and the " big picture, " but it does not provide the granular detail required to execute the specific tasks of a single story.

Product owner ' s priorities (Option C): Priorities determine the order in which work is done (sequence), but they do not define the technical or functional requirements needed to fulfill the story itself.

Per PMI standards, Acceptance Criteria are the primary source of detail in an adaptive environment that ensures the team has a shared understanding of the work requirements, allowing for accurate estimation and successful delivery.