The assist feature can be one of a few things, depending on how you set it:
If you set it to "TCS," then you've enabled the Traction Control System on every car, even those that don't have it in real life.
If you set it to "TCS ESC," then you've enabled both traction control and Electronic Stability Control on every car, even those that don't have those systems in real life.
If you set it to "Recommended," then traction control and stability control will only be enabled on the cars that have either or both systems in real life.
Then, of course, if you set it to "Off," it disables the traction and stability control on all cars, even those that have either or both systems in real life.