Maybe I could help to clarify.
Or make it worse.
With the PMA charging systems, the argument is MOSFET vs SCR.
The SCR design shunts (dumps) excess current to ground, back thru the PMA windings, creating heat.
The MOSFET design blocks excess current, producing less heat, but induces high voltages in the blocked PMA windings, challenging their dielectric coatings.
For our XS650s, with the field excited regulator, regulating the current of the rotor, the argument is usually Mechanical type B (relay-type) vs Solid-State type A.
The Mechanical (relay) type uses contacts that can get dirty or burned, and flitters between 3 power states (full, partial, grounded) which produces erratic voltages. This is the type B regulator that supplies power to the rotor on the 70-79 bikes.
The Solid-State type has no contacts and produces a much smoother output. This is the type A regulator that supplies a regulated ground to a powered rotor on the 80-84 bikes.
Now, for the variances.
You *can* find and use a Solid-State type B regulator to power the rotor on the 70-79 bikes.
You *can* reconfigure the early type B brush block to the later type A, and use a later Solid-State type A regulator. See the "3 nylon screw" mod.
Both type A and type B regulators can be packaged with a 3-phase rectifier, making a singular rec/reg unit. Normally a 7-wire device.
Now, with respect to the MOSFET labeling on the ElectrexWorld RR35 rec/reg.
Traditional Solid-State regulators use good ol' silicon transistors that reference a zener diode voltage to produce internal currents that are 'current gained' to control the output current of the rotor. The design risk is in using a batch of transistors that have varying hFE (gain).
A MOSFET regulator would use Field-Effect transistors that reference a zener diode voltage to produce internal voltages that are 'current gained' to control the output current of the rotor. This subtle difference would use less internal current, and run cooler. A bit risky back in the '80s, since MOSFETs were sensitive to moisture, spurious voltages, and static. Modern MOSFETs built to industrial/automotive specs (vs consumer grade) usually don't have that problem.
So, in summary, a MOSFET regulator for an excited field (rotor) type alternator could have a slight edge over the older transistor design. I personally feel that the MOSFET labeling in this particular instance is to capitalize on the marketing hoopla...