http://debian-xml-sgml.alioth.debian.org/xml-policy/xml-general-dir-file-policy.html


2.3. General Directory Naming and File Placement Policy

This section describes policy that is common to both infrastructure
and application directories, such as package directory layouts,
version-numbering directories, and the handling and definition of
customization packages.

2.3.1. Version-numbered Directories


When a package contains multiple versions of a resource, such as a
DTD, each version must be placed in a separate version-numbered
subdirectory. The docbook-xml package provides a good example.

The DTD files are placed in versioned subdirectories of
/usr/share/xml/docbook/schema/dtd.

Example 2.1. Version-numbered Directories for the DocBook XML DTD
/usr/share/xml/docbook/schema/dtd/ 	
3.0 (symlink to 3.1.7)
3.1 (symlink to 3.1.7)
3.1.7/

    *.dtd
    *.mod
    catalog.xml

4.0 (symlink to 4.1.2)
4.1 (symlink to 4.1.2)
4.1.1 (symlink to 4.1.2)
4.1.2/

    *.dtd
    *.mod
    catalog.xml

DTD packages that include customization layers, such as Simplified
DocBook (aka docbook-simple), should not install their files in this
hierarchy, they should instead install them in
/usr/share/xml/docbook/custom/simple. For more info, see the
Customization Packages section.

