It's a signal bender like the E-Manage.
It doesn't have full control of the motor unlike say an FC.
If I was to use one word to describe why an FC is better, it'd be "fine-tuning".
eg. you'll find it very hard to get the car anywhere near EPA with an S-AFC while the FC will make a better fist of it.
Saying that, I've seen the S-AFC control a car with a Z32, AFM and Trust internal gate turbo - just with horrible fuel use, emissions and performance was ok but a bit rough in delivery.
The FC cleared that up.
eg. there's a drop down menu for VG30 AFM and the injector size, plug and play as it were.
T.