Not switched as such, A1 and A terminals split electrical loads with a third bobbin providing current as well as voltage control for dynamo. Earlier two bobbin types just the latter with a single A terminal. This is why you need to join them together at the back when wiring in the simpler alternator with built in regulator, if retaining wiring to vehicle loads. The old regulator is then not used electrically at all other than an alternator to load junction point as shown in second diagram.
