An unknown shipper is a person or business that is not in the Known Shipper database maintained by the Transportation and Safety Administration (TSA). An unknown shipper cannot submit cargo to be shipped on an aircraft that carries passengers. It may only be shipped by cargo only aircraft and the labels and documents accompanying the shipment should state, "UNKNOWN SHIPPER. CARGO AIRCRAFT ONLY".
Personal effects are always considered "unknown cargo" even when the shipper is known, and can only travel by cargo aircraft unless they are travelling on the same passenger aircraft flight as the shipper.