Which one must be used matched output voltage or open circuit voltage?
You must use both values when designing a circuit around such a generator (or solar panels, alternator, etc)

Your circuit needs to safely withstand the open circuit voltage without damage.

But in operation, drawing the rated current from the generator, you can expect the matched voltage. If you're computing charging times, power ratings, efficiency, this is the value to use.

As the battery becomes fully charged, you will draw less than the rated current, then the voltage will rise towards the open circuit voltage as the current decreases.