Ah! You are exactly right! I wasn’t even thinking about backwash. So that one probably already had the plug inserted out of the box.One of those open 2 near the multi port is backwash right? The other one should say flow or have an arrow or something of that nature. It is the return to the pool. You can put a hose on the backwash side & run it out into your yard.
Also the sand # is approximate so just fill 2/3 to 3/4 of the way up. Kinda about where the lines are on it.