When you refer to 2 wire you are referring to ONE field wire and ONE charge wire.
This is typical of the "Round back" alternators
When you refer to 3 wire, you are referring to "isolated field" TWO field wires and ONE charge wire. This is typical of the "Square back" alternators.
FYI. At some point there were round back style alternators with isolated field (2 field wires)
(And yes an isolated field alternator can be converted to a non isolated field)
