The threshold refers to the maximum continuous steam output capacity of the boiler as designed and rated by the manufacturer, measured in tonnes per hour. Boilers rated above 45 t/h classify under 840211; those at or below that output classify under 840212. Manufacturer technical specifications or nameplate data are the standard reference for establishing this figure at entry.

Parts specifically designed for steam or vapor generating boilers of heading 8402 classify under 840290. Parts designed for central heating boilers of heading 8403 classify under 840390. If a part is genuinely interchangeable between both types, classification follows the heading whose boilers represent the principal use; a binding ruling is advisable for ambiguous components.