What does an 'U' response in AVS indicate?

In the context of Address Verification Service (AVS), a 'U' response indicates that information is unavailable for the transaction being processed. This response signifies that the AVS system could not retrieve the necessary address details to perform a verification; therefore, it cannot determine whether the provided address matches the records on file with the card issuer. This may occur due to various reasons, such as the issuer's systems being temporarily down or the address not being in their database.

Understanding this response is crucial for merchants and payment processors when evaluating risk. It reflects a need for caution, as the lack of address information means there is no validation available for the transaction, potentially increasing the risk of fraud. Employing this knowledge allows merchants to implement additional measures or controls when encountering an 'U' response.