In the message flow of a proxy service, you can assign XQuery expressions to message context variables, assign If...Then... actions based on the Boolean result of an XQuery expression, insert the result of an XQuery expression at an identified place relative to an XPath expression, specify the message context that you want to log through XQuery expressions on context variables, and so on.
The XQuery Expression Editor, the XQuery Condition Editor, and the XPath Expression Editor are available in the appropriate context in message flows to construct the kind of expression called for in the context.