Serna fully supports OASIS XML Catalogs (Committee Specification 06 Aug 2001).
On opening a document, Serna uses XML Catalogs, which contain paths to DTDs, Schemas, and stylesheets.
The path to an XML Catalog is specified in a corresponding document template.
If the path to an XML Catalog is not specified in a document template or you need to add some more XML Catalogs for all document types, you can do it in the Preferences dialog.