Heading 840810 is specifically reserved for marine propulsion applications, while heading 840820 covers vehicle propulsion for Chapter 87 vehicles. If a diesel engine is a dual-purpose design marketed for both uses, classification follows the primary intended application as documented by the importer. CBP may request technical literature, sales invoices, or end-user declarations to confirm the application when it is not self-evident.

Lines 8408201040 and 8408201080 are dedicated to engines for agricultural tractors, split at the 37.3 kW power threshold, while 8408202000 covers engines for road tractors, buses, automobiles, and trucks. An engine physically identical in design but sold to a tractor manufacturer versus a truck manufacturer would classify differently based on the documented end-use. Importers should retain purchase orders or OEM supply agreements confirming the vehicle application.