The SOAP Header element contains header information.
The optional SOAP Header element contains application-specific information (like authentication, payment, etc) about the SOAP message.
If the Header element is present, it must be the first child element of the Envelope element.
Note: All immediate child elements of the Header element must be namespace-qualified.
The example above contains a header with a "Trans" element, a "mustUnderstand" attribute with a value of 1, and a value of 234.
SOAP defines three attributes in the default namespace ("http://www.w3.org/2001/12/soap-envelope"). These attributes are: mustUnderstand, actor, and encodingStyle.
The attributes defined in the SOAP Header defines how a recipient should process the SOAP message.
The SOAP mustUnderstand attribute can be used to indicate whether a header entry is mandatory or optional for the recipient to process.
If you add mustUnderstand="1" to a child element of the Header element it 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.
A SOAP message may travel from a sender to a receiver by passing different endpoints along the message path. However, not all parts of a SOAP message may be intended for the ultimate endpoint, instead, it may be intended for one or more of the endpoints on the message path.
The SOAP actor attribute is used to address the Header element to a specific endpoint.
The encodingStyle attribute is used to define the data types used in the document. This attribute may appear on any SOAP element, and it will apply to that element's contents and all child elements.
A SOAP message has no default encoding.