In HL7, an ACK is the message sent from a receiving system to a sending system, signalling that data has been received successfully.
HL7 provides different codes that can be included with ACKs to state whether the data was:
- Received (AA),
- Received and processed (CA),
- Contained errors (AE), or
- Was outright rejected (AR).
ACKs with error codes are often referred to as NAKs or NACKs (negative-acknowledgements) though it’s not actually a different message type.
