From the AICPA Standards for Performing and Reporting on Peer Reviews:
The purpose of a letter of response is to describe the actions the firm has taken or will take, including the timing of the planned actions, to prevent a recurrence of each deficiency discussed in the report. If the reviewed firm disagrees with one or more of the deficiencies or recommendations in the report, the reviewed firm should contact the administering entity for assistance. If the firm still disagrees after contacting the administering entity, the firm’s response should describe the reasons for such disagreement. For more information related to disagreements, see paragraph 93 of the standards. The letter of response should be carefully prepared because of the important bearing it may have on the decisions reached in connection with acceptance of the report on the review (see paragraphs 136–140, “Accepting System and Engagement Reviews”). The letter of response should be submitted to the team captain for review and comment prior to the firm submitting the response to the administering entity. Illustrations of responses by reviewed firms who receive reports with a pass with deficiencies or fail disposition can be found in the AICPA Standards for Performing and Reporting on Peer Reviews.