There is other information about voltage regulators on the site, but since the old units are out of production, this information seems most appropriate.
Evidently Bosch has shut down the production line for the 25 and 30 ampere electro-mechanical voltage regulators. They have substituted a 'new generation' transistorized one made in Mexico (complete with nice instructions in Spanish). There are a couple of choices for model numbers, 099E and 099U. Not sure what the difference is - I'd guess the form factor - but this report is about the '9 190 040 099E'. Both are for VW from '68 on. I believe these will all be 30 ampere generators. I'm using it with my 25 ampere generator.
A 'voltage' regulator should add charge to the battery. It needs to limit the voltage so the battery does not over-charge and it needs to limit the current so the maximum energy limit from the generator is not exceeded. It may do this by comparing the battery voltage to the generator voltage and limiting the current - hence the regulator must be tailored to the maximum output from the generator. The other way for the regulator to work is to compare the voltage of the battery to a reference voltage and let the generator build up the voltage to that limit. If the voltage reference is just below what a fully topped off battery may be, then you will never try to over-charge the battery. It will at the same time provide some limit to the current to protect the generator.
It is also interesting that the generator light acts differently than before. At idle the generator light is quite bright. About 1200 RPM, the light goes out. Not dim, but out. Even with the lights on. This is different than the stock regulator where the light was dimly lit, particularly noticeable at night.
But do you risk burning out the generator with this thing? Apparently not. First it is recommended by Bosch, so they don't think it is a problem. Second, on the box, on the unit or the instructions there is no mention of current limits. I think they have added some electronics in the unit that measures battery voltage to much more accuracy than the old units and limit the current drawn as a result. Does this mean you can be safe in drawing too much current with your car with some combination of stereo, high powered head lights? The answer has to be no. I turned on the lights and did not see any increase in current drawn from the generator, but that was when it was drawing 10-12 amperes - enough for the car and the lights. But really, I don't have a way to test this case. Of course, my Optima battery is not a worn-out die-hard, so the regulator works well with other good components. Who knows what would happen otherwise.
It all seems good - better than the old one. I'll report back if there are any problems with it as I continue to use it.