The alternator you have uses a high side control. The 12v to the field is varied between 5v and 12v. The higher the voltage the more current the alternator can put out while maintaining a voltage in the 12 to 14v range.
The 2 field wire ( isolated field) use lowside control. One field wire has 12v from the battery (probably through the ignition switch but I don't recall) the other field wire runs to the electronic VR (triangular connector) and has the ground varied. The closer to full ground the closer to 12v the alternator will see, the more current the alternator can put out while maintaining a voltage in the 12 to 14v range