Any value can be rated in Shifts. Shifts are merely the unit of measurement. You can have Shifts of difficulty, shifts of effort, shifts of time, shifts of spell power, shifts of overflow and shifts of spin.
A note on the last two:
Overflow is a term used specifically in Dresden to discuss shifts beyond those required to succeed that can be spent discretionally. Spin is something else.
Spin is a special effect in some FATE games, (often added as an optional rule in an appendix or side bar) where defensive or, sometimes, maneuver rolls can "generate" a point of spin for every 3 shifts of overflow success that the player does not spend for effect. This spin can then be applied as a +1 bonus or a -1 penalty (as the player decides) on the very next roll in combat.
For example, let us imagine a scenario where your character, a simple shopkeeper, swung a broom handle at my character, a veteran soldier. You rolled a 1 total on your weapons roll. My defensive dice turn up entirely uninteresting, but still end up giving me a total of 5 on my defense. I defended against your attack and still had three Shifts left over, unused. I could then attack you on the following action, applying my spin as a +1 to my attack. Or, if the police officer intent on stopping your violence went next, instead, I could apply my +1 from spin to his attack roll against you.
Spin basically gives someone a minor control over combat for doing exceptionally well on a non-attack action.
Just an FYI.
