MSH segment
The MSH (Message Header Segment) defines the intent, source, destination, and some specifics of the syntax of a message. These are the required elements for POU:
| SEQ | LENGTH | Data Type | OPTIONAL | ELEMENT NAME | Infor POU Expected Value |
| 1 | 1 | ST | R | Field Separator | "|" |
| 2 | 4 | ST | R | Encoding Characters | "^~\&" |
| 3 | 180 | EI | O | Sending Application | SENDING_APP |
| 4 | 180 | EI | O | Sending Facility | SENDING_FAC |
| 9 | 7 | CM | R | Message Type | MESSAGE_TYPE |
| 10 | 20 | ST | R | Message Control ID | MESSAGE_CTRL_ID |
| 12 | 60 | VID | R | Version ID | HL7_VER |