There are two ways to set the value of a target element to a WebSphere® MQ constant, depending on whether the target element has an entry in the Map Script column of the Message Mapping editor Spreadsheet pane.
$mq:MQ_MSG_HEADER_LENGTH
$mq:MQMD_CURRENT_VERSION
When the map is saved a warning message is displayed if the expression entered for the WebSphere MQ constant is incorrect, for example, if the constant is not recognized. This is an example of the warning message: The target "$target/purchaseOrder/comment" is not referencing a valid variable.