title: what does it mean to "understand" a soap header tagged "mustunderstand"
body: <p>I am learning about SOAP. I see that some header elements have a "mustunderstand" attribute that can be set to true or false. But what does it mean to "understand" a soap message? To parse it without errors? I have found a few sites that <a href="http://msdn.microsoft.com/en-us/library/system.web.services.protocols.soapheader.mustunderstand.aspx" rel="noreferrer">don't</a> <a href="http://wso2.org/library/tutorials/understand-famous-did-not-understand-mustunderstand-header-s-error" rel="noreferrer">really</a> <a href="http://tutorials.jenkov.com/soap/header.html" rel="noreferrer">explain it</a>. </p>
  
  <p>Can somebody give me a rundown of what's going on?</p>