Confused: 2 suction openings in skimmers. One skimmer both open, second only one

[DigitallyChallenged]

I have an air entrapment problem with my shallow skimmer. To reduce water loss I decided to plug it besides having skimmer valve off.

I noticed that my deep skimmer has 2 2" threaded openings in the bottom but only one is open. The shallow skimmer has 2 2" threaded openings in the bottom.

Can someone educate me as to why there would be more than 1 suction opening per skimmer? What is the usual configuration?

 

[PrairieGirl]

My skimmer s have 2 openings. One goes to suction, one goes to a port in the wall just below the skimmer, I think to keep if from going dry if the water gets below the skimmer opening. not sure this is your setup or not.

 

[BTL512]

My second suction pipe goes to our water feature pump.

 

[pooldv]

The hole away from the pool is plumbed to the pump. The hole closer to the pool can either be blocked off, plumbed to a bottom drain or plumbed to an equalizer pipe in the pool wall directly below the skimmer.