Control Module For The Steering Wheel Angle Sensor (1999-2004): Function
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2006 Volvo S80, 2005 Volvo S80, 2004 Volvo S80, 2003 Volvo S80, and 2002 Volvo S80. However, it is still accessible from the selected car via links, so may be relevant.
See the DSTC (Dynamic stability and traction control) function section in BRAKE CONTROL SYSTEM - DESIGN AND FUNCTION for the brake control module or ABS control module.