The Flatten DTD command is enabled when a DTD is the active document. It creates a new flat DTD, removing parameter entities and producing a single DTD from a collection of modules. It also suppresses sections marked IGNORE and deletes unused parameter entities.
The command pops up a Save dialog, in which you select a location at which to save the generated DTD file. Click Save to carry out the conversion. The flattened DTD file is generated and opened in XMLSpy.