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GATT Article II provides that signatories may "bind" tariff duties by including them in their Schedules of tariff concessions, annexed to the General Agreement on Tariffs and Trade. Once a duty is bound, it may not be raised above that bound level without compensating affected parties. If the MFN applied duty is lower than the bound duty, the bound level of the duty is called a "ceiling" binding (see Ceiling Binding). If the binding does not cover all products in the tariff line the binding is "partial" and is usually identified by "ex" in the Schedules of tariff concessions.
In the IDB, bindings are recorded in the Textual Tariff (TT) file in the binding code / binding coverage (for partial binding) fields as follows:
B = fully bound,
BX = partially bound,
C= fully bound at ceiling level,
CX = partially bound at ceiling level,
D = Bound at different levels (mixture of fully bound items and items bound at ceiling levels),
DX = Partially bound at different levels (mixture of fully bound items and/or items bound at ceiling levels and unbound items),
U = unbound.