Volumetric efficiency, the hydraulic pump refers to the ratio of its actual flow to the theoretical flow.
The hydraulic motor refers to the ratio of the theoretical flow to the actual flow. Mechanical efficiency, the hydraulic pump refers to the ratio of the theoretical torque to the actual input torque.
The torque of the actual output of the hydraulic motor is the theoretical torque to overcome the torque after friction, so its mechanical efficiency is the ratio of the actual output torque to the theoretical torque. Total efficiency refers to the ratio of the output power of the pump (or motor) to the input power. The total efficiency equals the product of volumetric efficiency and mechanical efficiency.