With this option you can choose if you want to take geometrical nonlinearities into account. Linear analyses require less numerical effort and are sufficiently accurate for a large number of problems e.g. with small rotations and small strains. If you expect large deformations or large rotations you should consider to choose a nonlinear analysis. If you used the solver perspective (advanced option) and chose the Code Aster solver, the nonlinear option is only available if you set the nonlinear analysis property to true in the analysis type choice.