If you choose not to map message headers or the LocalEnvironment explicitly in your message map, the output message is produced with the same message headers as the input message. When you Populate the message map, the Properties folder for the source and target are displayed in the message map, with MessageSet and MessageType initially set based on the target message.
MessageFormat is set to the default wire format of the message set if the parser domain is MRM. The other properties are blank initially, and the message headers are copied from the input message.
Alternatively, if you choose to map any message headers or the LocalEnvironment in your message map, no message headers are copied from the input message. You must add mappings for these headers to ensure that the target message contains appropriate headers to make a valid output message.
To add message headers or other folders to a message map:
You can map multiple instances of a header by right-clicking on the header in the Message Mapping editor Spreadsheet pane and selecting Insert Before or Insert After. Select the header from the Insert Statement dialog.