A linear-acting hydraulic engine or motor (cylinder) produces straight-line force and displacement through a piston and rod assembly. A rotary hydraulic motor produces continuous rotational torque. The output motion — linear vs. rotational — is the definitive classification criterion, regardless of the fluid medium or operating pressure.

Importers should document bore diameter, stroke length, maximum operating pressure, rod diameter and material, and whether the cylinder is single- or double-acting. These parameters not only support classification but are also relevant if the product falls within the scope of any antidumping or countervailing duty orders covering hydraulic cylinders, where precise physical specifications determine scope inclusion or exclusion.