Its is often desired to break up a single big documents into parts for ease of maintenance and for code/markup reuse. Also sometimes it is required to include external files(text, source code, etc) into the document without having to format/copy it in the including document.
In order to use XInclude rewrite the document tag like this.
An external xml document can be included in the source xml document like this
<xi:include href="path-to-external-file" />
An external non-xml file(like text or source file) can be included like this. Note the parse="text" attribute.
<xi:include href="url-to-external-file" parse="text" />