Small spark-ignition engines not designed for aircraft, marine, or vehicle propulsion generally fall under 840790 (NESOI). The key distinction is the intended application: engines designed for vehicles fall under the displacement-based subheadings (840731–840734), while those for other uses, such as power tools or outdoor equipment, fall under the residual subheading.

Rotary engines are covered by this heading alongside reciprocating engines, but the displacement-based breakpoints in 840731–840734 are written for reciprocating piston engines. A rotary engine for vehicle use would typically be classified under 840734 or 840790 depending on its output and application; a binding ruling from CBP is advisable for non-standard configurations.