Here's an example that I found that is being used by another developer on our team:
The xsl file references this xml file, which is actually a combination of three other files, as I understand it. So I'm trying to do a similar thing myself, but I'm having issues. I may have to post a thread about that later.
<?xml version="1.0" encoding="utf-8"?>
<?xml-stylesheet type="text/xsl" href="es_related_report.xsl"?>
<!ENTITY related_codes SYSTEM "../qde_related_codes.xml" >
<!ENTITY type_codes SYSTEM "../typecodes.xml" >
<!ENTITY criteria SYSTEM "../qde_criteria.xml" >