This response is typically seen when the repeater does not recognize what was sent as a valid callsign routing command. This indicates normal repeater operation and if when using a repeater or reflector, can be treated as a response indicating that the station was heard, similar to the beep response of a traditional FM repeater.
The UR? response in D-STAR means that your signal reached the access repeater and your call was successfully sent from the destination repeater. This confirms that your transmission was received by the system. You should get a reply call from the station you are trying to contact, or hear other stations on the reflector you are linked to. If you do not get a UR? response, it means that your signal did not reach the access repeater, or the destination repeater did not recognize your call sign. You may need to check your settings (starting with verifying your registration if you have not yet done so) or try a different repeater or reflector.