Every HL7 message is part of a two-way communication system between two applications. A message that is sent to a system must somehow be acknowledged by the recipient. Acknowledgments, known in the HL7 world as ACKs, let the sending system know that their original message was successfully received and processed. This two-way communication is demonstrated in the following diagram:

If the message was successfully sent, a positive ACK message is sent to the sending system and the communication is over for that particular message. A positive ACK is indicated by using an AA (Application Acknowledgment) value in the first field of the MSA segment. This code indicates that the message has been received and processed without any issues.