Green car pass use for regular seats?

Hi there,

I'm looking to get a green car pass for 14 days coming up for the middle through the end of May. I'm not clear on whether a green car pass gives you the option of reserving green car seats AND the ability to just walk on to a normal seat? Or do you have to reserve a green car seat?

I ask because while I do plan to reserve travel most of the time, I would like the option of just deciding one morning to jump on a train without having to get a reservation :)

Thanks!