Typically, many schema and/or stylesheet distributions ship with one or more files that are intended as a means to customize the schema and/or stylesheets. These are to be treated as configuration files. An example here is the file dbgenent.mod included in the DocBook XML distribution.
Configuration files should keep their original, upstream names.
Configuration files should be placed in the directory /etc/xml/package. However, if package contains multiple versions, then the configuration files would be placed in version-numbered subdirectories: /etc/xml/package/version1, /etc/xml/package/version2, ..., etc.
The configuration file must also have a symbolic link in the package installation directory that points to the configuration file in /etc/xml/package.