When the ignition is turned on, voltage is supplied to terminal 15. But the generator is not working yet, so the de-energized contact D+ acts as a "minus". The control lamp lights up, because there is a difference between the voltage of the on-board network, which receives power from the battery, and the voltage of the generator, which is still not working.
When the engine starts and the generator reaches the charging speed, the voltage regulator connects the generator to the on-board electrical equipment. Now the positive current comes from terminal 15 and additionally from terminal D+. Due to this, there is no longer a difference in voltage, the charging indicator light goes out.
The control lamp is no longer needed for the so-called "pre-excitation" of the alternator; in the Audi A4 this already occurs during engine start-up via the 50A ignition switch terminal.
Charging does not always occur
The fact that the battery is being charged by the generator is not proven by the indicator light going out. It only means that there is no longer a difference in voltage between the battery and the generator. If, for example, all current consumers are switched on when the engine is idling, the indicator light does not light up, although more battery current is consumed than less powerful generators can provide: nevertheless, there is no difference in voltage compared to the battery.
(Information obtained from this resource: «AUDImanual.ru»)