Except for I think California, it means no payout whatsoever. Fun!
I've had unlimited policies before and I'm much happier having a specific number of days now that I use every year.
Banks often require that employees take vacation, typically around 2 weeks a year. An employee-focused software company should implement a similar policy of "required minimum vacation" [1].
The connections I have in Germany working in relatively "low-skill" fields are taking over 5 weeks a year of PTO. Even the most generous companies in the US are a joke by comparison to the most barebones companies in many EU nations. Without collective bargaining, we will always be far behind in benefits.