Skimmer Question

Can anyone tell me what the two pipes running down from my skimmers are? One seems to provide the suction when the pump is runnung, and the other one was closed off with a little cover. I let some DE get down there when I replenished the DE after I backwashed. Is that going to cause problems?

Is there ever any reason to uncover the hole, or to move the cover from one to the other?

Apparently, it's there for different configurations.
I would assume the most everyone uses the "basic" set up.
I would venture to say there's a good chance that nothing is connected to the skimmer under that plug.
See the attached link for some explanations.

Hayward Skimmer Manual

Thanks. This is helpful.