XMLTV comes with a mechanism to update fast changing supplementary files between releases.
The files are provided on a central webserver at http://supplement.xmltv.org. Convieniently the DTD is stored there too, even if it is not fast changing. This enables you to refer the DTD in exported XMLTV files.
Example: <!DOCTYPE tv SYSTEM "http://supplement.xmltv.org/xmltv.dtd">
Grabbers access supplementary files via the module XMLTV::Supplement(3pm) which takes care of fetching and caching.
Typical examples of supplementary files are mapping lists for channel ids, categories, etc.