Configuration files, including centralized catalogs.
|*.conf: generic configuration files|
|sgml-docbook.cat, tei.cat, ...: DTD-specific centralized catalogs|
|catalog: the super catalog|
Architecture-independent files used by SGML/XML computer programs: Open Catalogs (not the centralized ones), DTDs, entities, style sheets, and other declarative files, if any.
It is organized into DTD-specific subdirectories:
Data that are not DTD-specific go directly into /usr/share/sgml, preferably under their own directory.
At least for the present, all XML documents are also SGML documents, so it seems unnecessary to create /usr/share/xml and /etc/xml.