Duty is assessed on the value of the foreign processing — typically the cost of foreign labor, materials added abroad, and related charges — rather than on the full customs value of the returned article. The importer must establish the article's value at the time of export from the US (often supported by the export entry) and then document the foreign processing cost separately on the entry. The applicable duty rate is that of the article's underlying HTS classification, applied only to the foreign increment.

Engines are separately enumerated because they are a high-value, frequently cross-border-processed commodity in North American manufacturing, and CBP has historically scrutinized engine repair claims closely given the potential for misuse of the partial-duty provision. The dedicated line allows CBP to track and audit this commodity stream more precisely. Importers of engines should ensure repair documentation is especially detailed, as the line's specificity signals heightened compliance attention.