The DTD/Schema | Assign DTD... command is enabled when an XML file is active. It assigns a DTD to an XML document, thus allowing the document to be validated and enabling intelligent editing for the document. The command opens the Assign File dialog to let you specify the DTD file you wish to assign. You can also select a file via a global resource or a URL (click the Browse button) or a file in one of the open windows in XMLSpy (click the Window button). Note that you can make the path of the assigned DTD file relative by clicking the Make Path Relative To... check box. When you are done, your XML document will contain a DOCTYPE declaration that references the assigned DTD. The DOCTYPE declaration will look something like this:
<!DOCTYPE main SYSTEM "http://link.xmlspy.com/spyweb.dtd">
Please note: A DTD can be assigned to a new XML file at the time the file is created.