Basic question on drains

[Xentex]

This is undoubtedly an easy question...

Trying to get my pool working and not sure what I'm looking at here in the drains... The pool has two drains at the bottom of the deep end, both look like what's pictured below. There are two ports in each drain, one of which is plugged (the one on the bottom) and one of which is open (the one on the side with the hose going into it).

I'm not sure what's what. I expected to get in there and there would just be one port and it would be plugged. Now that I see two ports I'm not sure what's going on. There is also a spa with 4 drains, and all four of those look like this as well - side ports not plugged, bottom ports plugged. Looks like the same story in the skimmers as well. Two ports, one plugged, one open.

Pool was winterized before being closed, but that was more than three years ago under prior owner. I want to test the pumps, but don't want to try turning anything on until I figure out what's going on here.

Like I said, I'm sure this is an easy one, but not to me!

If that image isn't working maybe this:
Shared album - Mike Widner - Google Photos

 

[PoolguyinCT]

The plugged ports are hydro relief, leave the plugs in

 

[Xentex]

Perfect, thanks.

So when winterizing a pool people don't typically plug the drains? They just use the shutoff valves to keep the pump dry? I'm now wondering why the plumbing doesn't freeze and burst. Or why when I fill up the pump reservoir the water isn't flowing back to the drains.

 

[cfherrman]

Rv antifreeze

 

[PoolguyinCT]

Perfect, thanks.

So when winterizing a pool people don't typically plug the drains? They just use the shutoff valves to keep the pump dry? I'm now wondering why the plumbing doesn't freeze and burst. Or why when I fill up the pump reservoir the water isn't flowing back to the drains.

Antifreeze is heavier than water.

Airlock the drains to winterize, no antifreeze.