you need Revo to use the SPS3.
The SPS is a Serial Port Switch - it modifies the Revo settings on your ECU Map (within set boundaries)
SPS undetectable? The SPS device is a small plug type device - it only needs to be in the car when you want to make a change. 99% of the time mine is stored in my drawer in the house.
Go here for a better explanation, pics etc:
www.revotechnik.com
In short, you can use revo without an SPS device, but you cannot modify the settings (fuel type, boost, timing, ecu disable) without the SPS3.
I got mine 2nd hand for about £180.