Each HL7 message is of a particular message type. This HL7 message type indicates what health-related information is being provided in this message. The message type also determines what segments can be included as part of the message.
To determine the message type of an HL7 message, examine its MSH segment. The message type is normally the ninth field of this segment. For example, consider this MSH segment, which you have seen before:
Here, the HL7 message type is ADT^A04, which is “Register a Patient”.