The SOAP header is an optional element of a soap request or response. In the header section, meta information on the actual soap message can be set. These meta information can hold additional information about routing of the message, information on encryption or affiliation to a transaction.
The SOAP header can only exist once within a message and has to be the first element within the SOAP envelope. The header though can consist of multiple SOAP header elements. These header blocks are identified via a unique URI and, through that, can be assigned to specific nodes within the message structure. Through this mechanism, the SOAP message stays flexible and can be extended by specifying additional header blocks. The communicating partners do not need any specifications on these header blocks.

The content of these headers can be of any information. A typical usage in practice would be the transfer of security information or setting a transaction ID as the following example shows:

<?xml version="1.0" ?>
<env:Envelope xmlns:env="http://www.w3.org/2002/12/soap-envelope">
  <env:Header>
    <login:security xmlns:login="http://example.com" env:role="http://example.com/Login" env:mustUnderstand="true">
      <login:username>Simon Sample</login:username> 
      <login:password>aPassword</login:password>
    </login:security>
    <bridge:TransactionID xmlns:bridge="http://e2e.ch/bridge">dkdme44546kkjdkcvdfldkjvopinmoisf</bridge:TransactionID>
  </env:Header> 
  <env:Body > ... ... </env:Body>
</env:Envelope>

The SOAP message does not necessarily need to be sent from one sender to one receiver. The message can also reach its goal over intermediate stations. The elements can directly be addressed to these intermediate services assuming they understand and process the header information.