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. In the child element "address" it contains the identifier for the business partner which shall finally receive the document. In the child element "address" it contains the identifier for the business partner which sends the document. Usually an IBX Platform unique trading partner ID (TPID) that must be registered and reckognized to match a trading partner profile within IBX Connect.