When a Gold/Plat/EP member selects a seat, the seat next to them gets blocked. Someone else cannot generically choose that seat next to you and an AA agent or a travel agent sees that seat as a / instead of a * which means it's available. However, normal no-status AAdvantage members may not choose that seat whereas fellow Plat or EP or Oneworld Sapphire/Emerald member can still select that seat irrespective if there are other seats on the plane or not.

So it's not really that exclusive after all!