That's how externally regulated alternators work...here's a diagram of what happens.
To control the output voltage, and maintain the correct battery charge, the field winding current is varied. The regulator basically monitors battery voltage and when the battery voltage is low allows more current to the field. In this way the regulator acts like a rheostat and turns up the current as required.
The voltage at D+ is about the same as B+ because the voltage drop in the trio diodes (0.6v per diode IIRC) is almost the same as that in the larger rectifier diodes (they convert the alternating current into direct current).
If battery voltage drops, the regulator circuitry senses that fact (at D+) and increases the current flowing into DF until battery voltage is restored. Most regulators also include some form of temperature compensation. A cold battery requires slightly higher voltage to fully charge. A temperature sensing element in the regulator increases output voltage at low temperatures.
I know - this is still Greek...but this is how it works.