As user stories are completed (delivered) in the BACKLOG, it is now available for the Product Owner to review, test, and accept. It is important for the Product Owner to complete this in a timely manner to keep the velocity calculation as accurate as possible.
This completion, delivery, review, and acceptance of stories continues through out the project. Pivotal Tracker is effective in tracking, taking control of, and reporting the progress of work throughout time.
Any stories that do not fall within the expected action can be rejected, with proper reason indicated in the rejection. The developer team can then revisit the story, make the necessary fixes, and re-deliver it for another round of review and acceptance.