On a Super Duty I'd have to think an alternator is stated per SAE.
The specification for alternators is SAE J56. It's basically a standardized way to test and mark so the current is given at uniform RPM, voltage and temperature. For the purpose of marking the SAE temperature is defined as 23°C ± 5ºC, test voltage is 13.5, the "idle" is defined as 1,500 RPM and "rated" is 6,000 RPM.
So a hypothetical alternator would be marked like 200/377/13.5V in your example, meaning idle/rated/test voltage at the test temperature.
The typical way to show an alternator is to give an ambient (23ºC) and hot (93ºC) curve over a range of RPMs and probably at a higher voltage.