DF(M) stands for Digital Field Monitor. Every alternator brand has a different abbreviation for the DF(M) connection, for example: FR (Field Return), DF (Digital Field), DFM (See above), M (Monitor), LI (Load Indicator). They all function in the same way.
These are the following functions. About the DFM connection there is a positive and negative measurement and both work with a block pulse. When the alternator load increases the block pulse depending on the car application becomes wider or smaller. This is measured in % also called PWM(Pulse Width Modulation). The car ECU then knows what the load is at a specific moment during charging. If the load is to high the car ECU can shut down some car accessories and or increase the idle speed.
DFM (M, FR, DF, LI, F) is a block signal (information) that is sent from the alternator to the car ECU. It shows the load level of the rotor of the alternator, also known as electromagnetic force. This has a direct influence on the produced energy of the alternator.
The voltage is regulated by turning on the rotor current with a frequency of eg.150HZ, which changes the electromagnetic force. The longer the duration time of the current, the wider the duty cycle will be.