The document() function finds an external XML document by resolving a URI reference, parses the XML into a tree, and returns the root node.
node-set document(string/node-set, node-set[optional])
Either a string or a node-set in which each nodes' string value is a URI, and, optionally, a node-set specifying a base URI to be used in resolving any relative URI found in the first argument. If no base URI is specified, and the argument is a string, the base URI is the same as the base URI of the stylesheet element from which it was called. If the argument is a node-set, the base URI is the base URI of the supplied node-set. If the argument is an empty string ("), the function parses the stylesheet itself.
Defined: XSLT, section 12.1
