Catalogs can also be done in a modular and portable way.  For example,
a set of DTDs can be bundled with a catalog file that references all
the resources in it using relative system identifiers.  Then this set
of DTDs, along with its catalog, can be relocated anywhere you want.
To plug it in to a system, just add a single "nextCatalog" entry in
the main catalog file that your system uses.