ABS / TC 5.3 (4-channel system) system description: Function

Driving-stability control: 

If Traction Control (TC) detects that a certain speed difference between the wheels has been exceeded (wheel spin), engine power is automatically reduced.

Brake control: 

In addition to reducing the engine power (driving-stability control), the TC (via the ABD) brakes the drive wheel that is spinning.

Since this control requires the drive wheels to be individually controlled, the ABS/TC system is a 4-channel system.