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 <jan@iptel.org>