During a project, it's important that everyone understand the accepted standards of quality and completeness, and precisely how strictly the requirements will be met. Thus, it's part of your role to help everyone stay on track by continually refining, defining, and communicating the acceptance standards
. Of course, during any project the acceptance criteria are bound to change. For example, the early acceptance criteria for requirements might specify that broad coverage is more important than deep detail ("an inch deep, a mile wide"). However, the details almost always become more important as a project matures. The important thing is to have all of the criteria clearly spelled out in advance, so that everyone can work toward the same definition of success.
For example, the criteria for handing off a requirements document may be as tightly defined as the number of open issues allowed, the degree of risk associated with the open issues, or the thoroughness and accuracy of the requirements statements. The criteria for vendor selection or the end solution may be as loosely defined as that all critical or high-level requirements are met, and that the mid-level requirements have a workaround.