This is an older style, externally regulated alternator. The large terminal goes direct to the battery +. It should be labeled "B+" near the terminal.
The F terminal is for the field. This wire generally goes through a switch on the ignition so the field is not energized all the time, and from there goes to the regulating relay. The R terminal goes to the R terminal of the external relay regulator.
The R wire senses the current amperage load on the alternator. There is another wire from the B+ near the battery that goes into the relay, to sense the voltage provided to the loads. The relay/regulator then adjusts the voltage of the field in the alternator to increase or decrease the output of the alternator to maintain battery voltage supplied.
The sensing of the battery current load will lower the voltage on the output of the alternator. The relay/regulator will compare the sensed voltage of the load, and the current output of the alternator, and depending on the comparison, the voltage in the field wire will increase to add amperage out, or it will decrease field voltage to limit the amperage supplied and thus maintain proper battery charge voltage of apprx 14.7 VDC.
If you are going to replace it, I would suggest you check the ext relay for operation, and take the alternator to a starter/alt specialty shop for testing. If you replace the alt and the relay is not working due to the contacts being shorted, or open, or damaged, you'll still have the same problem. You could also replace the alt with a similar case unit that is a one-wire which is internally regulated. The only wire it needs is the large B+ and maybe a ground to the engine.
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