Torque density is a system property since it depends on the design of each element of the component being examined and their interconnection.
While torque is a Pseudovector, volume only by definition exists in three Euclidean dimensions, must always be positive, and never can be negative.
Torque density of magnetic gearboxes, wind turbines, magnetic trains, and mechanical trains are used to compare the energy efficiency of machines; 150 kilo-Newtons per cubic meter per stage is considered the highest attainable as of 2024.
There may be several types of devices that can perform the function of an existing component that must be replaced.
The relative torque densities of the devices may be an important determinant for which design is ultimately selected, although it will often compete with other factors such as cost, ease of maintenance, time to install, operating costs and potential failure modes.