The motor in question is a three phase permanent magnet synchronous machine. The main issue in servo motor design is torque quality. Low inertia and low price are also two important aspects. If the torque and EMF constants vary, the gain in the control loop of the torque will vary, making it difficult to get exact control in the position loop of, for example, the arm of a industrial robot. It will also necessitate a higher rating of the power electronics.
Torque quality is achieved by careful design of the magnet shape and the windings.
The finite element method, FEM, is a very powerful tool to attain this goal. The selection of magnet material is also important, as KT and KE will vary if the remanent flux-density of the magnets vary with temperature. As magnets are expensive, a low magnet volume is important to keep the price of the machine low.