Linearity and non linearity has to do with whether the stiffness of the support is a function of displacement. Say, for ex: if a pipe lifts off a +Y support, will it act as a restraint? Obviously no.Similarly for a guide with gap, if the gap closes, the guide takes load, else it does not. Regarding friction, it is well explained in the CAESAR II technical reference manual.Support non linearity is also known as contact non linearity. The two other non linearities are material ( for ex: plasticity) and geometric ( like a volter's pole).

Regards