Originally Posted by AutoSlash

It includes anyone authorized to drive the rental car, whether that's a spouse that is automatically included with a company like Avis, Budget, Enterprise, Alamo, etc. by their contract terms, or by local law (like in California), or when that person was explicitly added to the rental contract (often for an additional fee). If the additional driver is legally allowed to drive the rental, then they are automatically covered by any/all insurance, regardless of the source.