...specifies a processing model and syntax for general purpose inclusion. Inclusion is accomplished by merging a number of XML information sets into a single composite infoset...
Note that JAXP 1.4 also supports XInclude, and I assume that there is a way to use JAXP 1.4 with JAXB to accomplish the same (Kohsuke?), but JAXP 1.4 is in Mustang, while GlassFish only relies on Tiger.