A client representation letter is mandatory at the end of every engagement. Basically, this report states that management takes responsibility for the financial statements. In the course of an audit, if the client does not hand an auditor a signed representation letter, this becomes an automatic scope limitation resulting in a disclaimer of opinion.
Here is a sample of a representation letter:
http://www.vscpa.com/Content/peer_review/engagement_reviews/sample_rep_letter.aspx