Modularization of XHTML - Building DTD Modules
XHTML modules are implemented as DTD fragments. When these fragments are assembled in a specific manner (described in Developing DTDs with defined and extended modules), the resulting DTD is a representation of a complete document type.
http://mirrors.webthing.com/view=Medium/www.w3.org/TR/xhtml-modularization/dtd_module_rules.html