Card Markup Language is an XML based file format. This format is fully supported by Flash Card Master.
If you would like to manually edit the CML files, a document type definition has been written in RelaxNG, available at http://flashcardmaster.sourceforge.net/cardml/1.0/relaxng. If you prefer an XSD schema is also available at http://flashcardmaster.sourceforge.net/cardml/1.0/schema.