So what's with all the chambers?

[DarkHorseMBA]

I'm working on refurbishing my first reef tank, there is another post about that here. The tank has a built in sump/filter, its in the back of the tank. I thought I knew how sumps worked, but this has so many baffles and chambers. I've attached a photo and diagram, I'll try to explain what I know or think things are, maybe someone with a patient disposition can help explain some of this to me.

Attachment 9272

Between each chamber there is a black pvc egg crate type grid.

1. This is the overflow water, water enters here and once the chamber is filled the water flows into chamber 2.

2. These seems to be where I would put a filter or sock to catch fish waste? The water then flows/trickles into chamber 3.

3. Is there where I would put carbon? Then the water flows into what looks like a bubble trap, and into mystery chamber 4.

4. I have no idea what might happens here, the tube at the top is on the outside of the tank, and there is dipstick looking thing with a green tip at one end an acrylic tube on the other. The water flows from here into what looks like another bubble trap, and overflows into chamber 5.

5. there is a removable acrylic plate here with drilled holes, just seems to be laying there, no sure it belongs. Water passes into chamber 6.

6. I guess bio balls or refugium plants go here?, water flows to chamber 7.

7. Bio balls, if there are plants in #6.

Finally out to the large chamber to be pumped back into the tank. The tube shown in on the inside of the sump/filter, unlike the other tube.

So am I totally of base here? Any help would be appreciated.

DarkHorse