In IzPack 4 (probably functional until 5.0.6), there has been supported a special tag for XIncludes - <xfragment>, which allowed to include XML fragments without a root element (which is actual not "well-formed" XML, therefore the embedding <xfragment> tag).
See this discussion for the original intention.
There is not just a lack of documentation but I'd replace it by the "official" XInclude supported enhanced by XPointers like documented by the W3C. It seems there isn't that much people actually utilizing it, because it isn't neither compatible with the XSD validation we have activated already since 5.0.7.
I'll simply drop this. Howgh