The XML subset of SGML still includes most forms of entity usage SGML has, including external general entities as described by grandparent. XInclude can include any fragment not just a complete document, but apart from that was redundant, and what remains of XInclude in HTML today (<svg href=...>) doesnt't make use of fragments and also does away with the xinclude and other namespaces. For reusing fragments OTOH, SVG has the more specific <use href=...> construct. XInclude also really worked bad in the presence of XML Schema.