**TO ALL LEXUS OWNERS**why are there two disc slots behind the navigation on the lexus es330?

to any owners of the lexus es330 or i'm guessing any lexus that has navigation, why are there two slots to put in discs? i mean the 6 cd changer is in the arm rest and behind the screen is for the navigation disc but then there is one more slot to put another disc...........is it a DVD player??? i dont wanna mess with it until i get a good answer lol (the owners manual is useless)