Cadifra UML Editor files (*.cdd) are stored in XML format. The schema is available as a zipped archive:
Schema1.3.zip (6'359 bytes)
The schema language we use is RELAX NG, Compact Syntax.
A good starting point to read about the RELAX NG Compact Syntax notation is the RELAX NG Compact Syntax Tutorial. This Tutorial does not assume prior knowledge of the XML syntax.