I usually leave it at default while constructing circuit in Cadence Virtuoso. I didn't really understand what it is or why it is used for.
In Global Model files section I have NN, SS, SF, FS, FF , etc. in drop down list. What does that mean?
Model files often have "sections" to allow multiple blocks of different models to co-exist in the same file. The idea is that you then include a section of that file when you simulate. Usually this is done to either indicate a specific type of component, and more specifically to allow you to pick of a number of different process "corners". The terminology NN, SS etc normally is a character for nmos and pmos, with N=Nominal, S=Slow and F=Fast - so SF means slow nmos, fast pmos. You're picking models from a different region of the process spec to allow you to uncover whether your circuit is likely to continue working and perform well enough as the process varies.
Hope that helps,