1.2. R001--SGML Directory layout

/etc/sgml/

Configuration files, including centralized catalogs.

It includes:

*.conf: generic configuration files
sgml-docbook.cat, tei.cat, ...: DTD-specific centralized catalogs
catalog: the super catalog
...

/usr/share/sgml/

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:

docbook/
tei/
html/
...

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.