The mustUnderstand attribute in a SOAP header block indicates whether the recipient must process the header block. The recipient is indicated by the actor element.

In the syntax for the mustUnderstand attribute, 1 or true indicates that the receiver must recognize the element and process it. Elements not annotated with the mustUnderstand attribute, or those that contain a value of 0 or false, are optional and need not be processed by the recipient.

In the code example, the mustUnderstand attribute is set to 1. Therefore, the recipient of this message must process the Header block.