Generally The nova C plugs are for offroad use and F plugs are for onroad use.

C = Conical (Is there such a thing as non-conical plug??)
6 - wire
T = turbo
G - gold
F = Cold C = Hot (pin size and body)

You should think of C & F in terms of

O.S P3/4/5 vs OS RP6/7/8.