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The XPath parameter expects the XPath query string. XPath looks much like a file path, drilling down into XML elements until you find the one you're looking for. XPath is far too broad a subject to teach here, but w3schools.com has excellent tutorials here.
Once you understand the XPath query language, simple pass the XML data in question and a query to extract the data you want. To get only an element's contents, append the query with "/text()".
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