When you program the DS64, you must program all four outputs, even if they are not connected to anything. A "Virtual" top address can be used to avoid conflicts with actual turnout addresses and other routes.
To create a virtual top address use a switch address for your top address that is not used to operate an actual turnout on the layout. The advantage of this is that you can consistently use either the "thrown" or “closed” command to operate all routes, making it easier to remember which position goes with which route to make them work. Since DS64 routes can only be 8 addresses deep, if you use a "virtual" top address, you will only be able to operate 7 actual turnouts with each route.