Dynamics in CAESAR II are "linear", due to the Eigen-solution. Therefore any non-linear restraints you may have in the model (such as +Ys or gaps) must be linearized. CAESAR II can do this for you by referencing an previously solved static load case.

Typically you want to reference your dominant operating case, since you're hopefully in that situation most of the time. CAESAR II evaluates the non-linear restraints in the referenced case and either (a) converts them to linear restraints if they are active, or (b) removes them if they are inactive.