A Pro Cab is only a Cab / Throttle. It requires a DCC system to function.
The Power Cab is a self-contained DCC system in one package. It is a Cab / Throttle, it is a DCC Command Station, it is a Booster. All three in one package.
*** Note: When you use a Power Cab with a booster (like the SB5) it works as a Pro Cab.