A unique messageID generated by the sender and used to check for duplicates received in IBX Connect. Mandatory: the date of transmission to IBX Connect. Optional. Not used for any business logic. Identifies the document type of the payload. The SOAP mustUnderstand attribute indicates whether a header entry is mandatory or optional for the recipient to process. If mustUnderstand="1" or "true" then this indicates that the receiver processing the Header must recognize the element. If the receiver does not recognize the element it will fail when processing the Header.