If you select this item, Creo Simulate automatically selects With Rigid Mode Search and looks for and reports rigid body modes for an unconstrained analysis.

You might select this option if you want to examine rigid body vibrations.

If your model contains point-to-ground springs, you can select Unconstrained separately from With Rigid Mode Search. Point-to-ground springs are the equivalent of constraints. You still have the option of selecting With Rigid Mode Search, but you do not have to select it as long as the springs sufficiently constrain your model.