Confirmation letters are generally considered the most reliable source of documentary evidence because they involve direct verification of information with an independent third party. This type of evidence is less prone to bias or manipulation compared to internal documents like policy statements or remittance advices.
IIA Reference:
IIA Standard 2310: Identifying Information requires that internal auditors gather sufficient, reliable, relevant, and useful information to support engagement results. Third-party confirmations, such as confirmation letters, are considered highly reliable because they come from independent sources.
The Practice Guide on Audit Evidence highlights that external confirmations provide strong evidence of the accuracy of the information being audited.