SER supports multiple database servers, at the moment of writing this text
MySQL, PostgreSQL, dbtext (aka plain text files), and Oracle databases are
supported. Maintaining database schema for each database separately would be
tedious job, therefore SER keeps the database schema in an XML-based format
and schema definitions for the database servers are generated automatically
using stylesheets.
This directory contains DTD (Document Type Definition) files describing the
structure of the XML-based database description language and XSL stylesheets
used to generate database schema files for all the database servers.
-- Jan Janak