What does it mean that Zuck has 3 seats -- one empty, one he holds, one by Andressen -- does Andressen get to vote as Andressen wishes, or does he have to vote

What does it mean that Zuck has 3 seats -- one empty, one he holds, one by Andressen -- does Andressen get to vote as Andressen wishes, or does he have to vote Zuck wishes, or does Zuck effectively get to cast all three votes? How do these board seats relate to the preferred boat seats?