Sump layout cad lights 70

[NY_Sea]

Morning.
I was wondering if any one could explain this sump layout to me. I am trying to reconfigure this sump and re plumb which is becoming a huge pain.

I have never given much thought to this design from cad lights and am now questioning what these chambers were meant for.
1 st is skimmer obviously. Second is ?.. third is refugium. And last is return.

I don’t understand why they have 2 chambers after the skimmer.

Any ideas?
i have been running it for years with live rock in second and chaeto in 3rd.

my sump collects a ton of detritus so I’m trying to brainstorm a better layout With more flow.
I could put the filter cups in skimmer area but it’s tight.

Also. Does anyone see an issue if I install filter cups and drain my overflow into second chamber. Or maybe split half to skimmer and half to chamber 2?

 

[dbowman5]

i see ledges in the second chamber, this could be for filtration either socks on the top or perforated sheets on the top and bottom to lay media bags or blocks on. i take it that the picture was taken with the pump off, otherwise the level looks too high for the weirs.