Acknowledgment messages may be defined on an application basis. However the simple general acknowledgment message (ACK) may be used where the application does not define a special message (application level acknowledgment) and in other cases as described in Section 2.12.1, "Original and enhanced processing rules." The MCF message is included only for backward compatibility with HL7 Version 2.1 (see Section 2.12.2, "Application (level 7) processing rules, deferred processing xe "Processing rules: application" xe "Processing rules: deferred" two phase reply (original acknowledgment mode only)").