What is an 'A' and 'B' circuit Voltage Regulator in a Auto Alternator

When you have an "A" circuit type voltage regulator means full power is sent to one brush while the voltage regulator controls the output by varying the ground to the other brush.
It does not matter which wire you run to the regulator field connection and which one you ground, you can swap them around.Early GM internal regulated alternators such as the 10si used the "A" circuit voltage regulators

The "B" circuit type voltage regulator grounds one brush inside the alternator. Then the voltage regulator controls the positive power to the other brush which runs to the rotor coil inside the alternator. Control of this positive power into the rotor coil turns the alternator on and off.
Examples: Ford used "B" circuit type voltage regulators in early Motorcraft external regulated 1G alternators . Later Ford units such as the 3G, 4G, and 6G alternators use "A" circuit voltage regulators.