The MSH segment is the MeSsage Header segment. Every HL7 message starts with an MSH segment. This segment contains the following information that’s relevant to an ACK message:
|MSH.3 – MSH.6||Sending Application (MSH.3), Sending Facility (MSH.4), Receiving Application (MSH.5), Receiving Facility (MSH.6)||Indicates what application generated the message, and what facility generated the message.|
|MSH.10||Message Control ID||Unique ID for the message being sent. This is used by the receiving system to distinguish between messages being sent to them.|
|MSH.12||Version ID||Allows the sending system to indicate which version of the HL7 standard that their message complies with.|
For the inbound parsing of each message you want to acknowledge, it is important to store the message control ID and possibly other parameters like the sender, version, etc.