Standard: http://www.w3.org/TR/xslt#include

include

Attributes: href

Standard excerpt:

The xsl:include element is only allowed as a top-level element. The inclusion works at the XML tree level. The resource located by the href attribute value is parsed as an XML document, and the children of the xsl:stylesheet element in this document replace the xsl:include element in the including document. The fact that template rules or definitions are included does not affect the way they are processed. It is an error if a stylesheet directly or indirectly includes itself.