"F" is the load applied to the support, "mu" is the coefficient of friction you defined for that support back in the Static Input. "Fact" is the value you define on the "Control Parameter" dialog.

This factor is actually a "fine tuning" control, to help with the accuracy of the solution. The problem is you can't use this unless you're trying to match field results. For example, you have a harmonic loading induced by equipment and you have measured the field response. Once your model is matching closely with those field results, you can use "Fact" to tweak the results to make things closer.