Over the years there have been many ways to accomplish the same thing. The standard was that you had one wire to power the field and the ground at the other end of the field was built into the generator or alternator and that was it. All control was done before the generator/alternator. Now, with the computer doing the control AFTER the alternator, you need a two wire field. Full power always goes into the field on one wire and it comes back out (insulated) and goes to the computer and it varies the current flowq and completes the ground there. The newer two wire alternator can be used on the older system by just grounding the field coming back out with a bolt or a jumper wire. Not that big of an issue.