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Note: For the general acknowledgment (ACK) message, the value of MSH-9-2-Trigger event is equal to the value of MSH-9-2-Trigger event in the query message being acknowledged. The value of MSH-9-3-Message structure for the general acknowledgment message is always ACK.
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[1]HL7 Messaging Standard Version 2.4, 2.3.2 Acknowledgments: original mode, 2000