Typically, many DTDs and/or stylesheet distributions ship with one or more files that are intended as a means to customize the DTD and/or stylesheets. These are to be treated as configuration files. An example here is the file dbgenent.mod included in the DocBook SGML distribution.
These configuration files should be placed in /etc/sgml/package/. If package contains multiple versions, then the configuration file would be placed in a version-numbered subdirectory: /etc/sgml/package/VERSION/.
The configuration file must also have a symbolic link in the package installation directory that points to the configuration file in /etc/sgml/package/