That's a weird drilling. The 2 in the center of the tank would be for returns, and one of the two smaller holes would be for return as the hole drilled in the side of the overflow has to be a return line or else there is no use drilling a big hole in a tooth overflow.
I would say it's almost designed for 2 sumps, 2 drains to go to two sump and 4 returns. I don't know a lot about reef tanks and if it's worth it having 2 different sump or a sump a filter going to a calcium reactor or a refugium or something instead, only thing I can think of.
Guess you could plumb one of the big returns directly out for water changes too. Turn a ball valve and drain water right from the tank but that would only work for the amount of water in the overflow, so I dunno. lol.
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.
Sump Pimp #2
My Tanks and my shrimps