The nominal rating for all alternators is either 12V or 24V. This is nothing more than the nominal rating of the battery system the alt will be working with. Just like 12V nominal solar panels that put out 19V.. You can even get higher voltage alternators too.
Most every car alternator out there is tested to SAE standards, specifically J56 (I have J56 somewhere in my computer but it would take longer to find it than type this). Car alts are almost all rated at 13.5V under specific J56 test conditions. However J56 allows for any manufacturer to rate their alt at what ever voltage they want to, provided the power curves clearly state the rated voltage. For example open any Yanmar technical manual and you will see the Hitachi alts are specified at 13.5V. Hitachi also adds the temperature compensation factor to comply with SAE requirements because these are first and foremost automotive alternators.
Typical Hitachi Alt Specs:
Current Rating Curve 13.5V
Regulated Voltage 14.5V (+/- 0.3V) at 20C (full battery)
Temperature Gradient -0.01V/C
The temp gradient for Hitachi alts means that for every degree of alt temp above 68F/20C it reduces the max absorption/regulated voltage by -0.01V. So when the alt gets to 200F the reduction in max regulated voltage will be approx -1.32V. If we started with 14.5V then the new regulated voltage is now just 13.18V!!! In most sailboat engine bays 200F is pretty common for an alternator. This is why self protective, temp compensated auto-based alts such as the Hitachi's are such poor performers on deep cycle banks in sailboats...
High output manufacturers are usually rating their alts at 14.0V + depending upon the manufacturer and don't generally follow J56 because these alts are not being used on cars and need to be tested for deep-cycling high output situations.
HO alts still need current limiting, in certain applications such as large flooded banks, most AGM and many GEL installs, but this is done very differently from output reductions by limiting voltage like Hitachi does on Yanmars.
With an external Balmar reg you can set a 100A alternator up to only ever produce 80A. However at 80A you can still hit your 14.6V or what ever absorption voltage your battery needs. The output current of the alt is not constrained by limiting regulation voltage output but rather reducing the field current/voltage to the alternator brushes. A HUGE difference in battery charging performance.
There is no industry standard for aftermarket high output alternators thus every manufacturer rates them & tests them slightly differently. I know that Balmar tests their alts with their own regulators into a LARGE bank of depleted AGM batteries to simulate real world environments. Electromaax does the same. My alternators are tested to 14.0V into a depleted battery load using external regulation as well. They are also hot tested and both hot and cold rated at alt shaft RPM. Each manufacturer however temp rates them differently. For some cold means 122F and for others it means 80F.. For some hot means 200F and for others 225F....
So the real answer is that unless you buy a car alternator, and almost all are rated at 13.5V, then you need to ask the manufacturer at what voltage their power curves are done.. Most all of the HO alts I have installed are conservatively rated. Some car based alts, like the Hitachi alts, ship with temp compensated dumb regulators to protect the alternator NOT your battery bank.....
I have yet to see one HO alt in full bulk mode that can not exceed its cold rating by 5-20A. Once they heat up they usually match the hot ratings quite closely..
Bulk mode performance is all that really matters and that is usually at voltages well below 14.0V... Once the batteries come up to target voltage the alt is not putting out the max it can anyway...