As default, the nonlinear bearing stiffness is considered for rolling bearings. For large systems this option can be cleared to decrease calculation time. Without this option, bearings are considered as rigid. The option is available for each shaft, so it can be set were bearing stiffness is important and can be cleared on other shafts.
If bearings with contact angle are used the nonlinear stiffness matrix should be considered to take into account the coupling of axial and radial loads.