2012 Audi A6 / S6 — Owner's Manual
Signal lights are built into both outside mirrors <=> fig.88. The signal light on the left outside mirror assists when moving over into the left lane and the signal light on the right outside mirror assists when moving over into the right lane. The signal light comes on to tell you that side assist has detected a vehicle on that side and that the position of this other vehicle should be taken into account if you were to change lanes. The informational stage signal is designed so that you notice it only when you are looking in the outside mirror.
Side assist compares the distance and difference in speed of detected vehicles with the speed of your vehicle. Whenever the difference in speed and distance is registered as notable if you were to change lanes, a signal light appears on the respective mirror. The signal can light up if you a repassed by a vehicle or if you are passing another vehicle. If you pass another vehicle slowly (difference in speed less than 9 mph, or 15 km/h), the signal light will come on as soon as the other vehicle is in your blind spot and is detected by side assist.
When the difference in speed is greater, the signal light will not come on.