A customs duty rate is unbound if it was never subject to a tariff concession during any GATT round of tariff negotiations (also see BINDING) .

The quotient showing the total value of a particular trade flow during a specified period divided by its volume. Unit values are often reflected in international trade statistics instead of prices.  

Source: http://usinfo.state.gov/products/pubs/trade/glosssz.htm#s

Multilateral trade negotiations launched at Punta del Este, Uruguay in September 1986 and concluded in Geneva in December 1993. Signed by Ministers in Marrakesh, Morocco, in April 1994.

The main outcomes of the Uruguay Round are the establishment of the WTO and multilateral trade agreements:  the General Agreement on Trade in Services (GATS), the Agreement on Trade-Related Aspects of Intellectual Property Rights (TRIPS), the Agreement on Agriculture; and the strengthened dispute settlement system.