The MSH segment is the MeSsage Header segment. Every HL7 message starts with an MSH segment. This segment has the header information about the sending and receiving application and facility. It also contains the version of each message. More importantly, it has the Message Control ID of the message, which is the unique ID of each message. 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.
