The DTD/Schema | Include another DTD... command allows you to include another Document Type Definition (DTD) or external parsed entity into the internal subset of a document type definition, or in any DTD document. This is done by defining a corresponding external parsed entity declaration and using that entity in the following line:
<!ENTITY % navigation.dtd SYSTEM "S:\xml\navigation.dtd">
The command opens the Assign File dialog to let you specify the DTD file you want to include in your DTD.
Please note: This command is enabled in Grid View only.