Looking a the text, it covers everyone in the vehicle for admission wherever the charge is per vehicle regardless of number of occupants (up to a resonable limit).

The non-relative charge only comes when the admission is per person, and I suspect charged only at places where you have to enter on foot.

If you are going to National Parks only and not to any Bureau of Land Management, Fish & Wildlife Service and/or Forest Service pay facilities, the National Parks Pass may be a better option. I also see that there are regional annual passes which may be better if you don't plan on using the pass in other parts of the U.S. within a year of issuance.