It’s your overflow box where filtration takes place often housing media and for servicing tank
I’ve noticed when I do fill it to high it flows into the boxes so I’m assuming, and it is dry at the moment I haven’t filled it all the way due to I don’t want water just sitting and it cause issuesFirst, welcome to R2R! The black box looks a little strange to me. It almost looks like a self-built AIO section. Does it have overflow slots to fill it? It looks dry in your photo.
Then it is an AIO (All In One) section. It is basically a sump that is in the tank instead of plumbed separately. It is where you put your filtration media instead of using a HOB system or canister or external sump. You will need a return pump in the last section of the chambers that should have a hole back into the tank, hopefully with an already installed bulkhead. Once the tank is full enough, water will start to flow into the first chamber of the AIO, it usually has a series of slots to take in the water but keep fish out. That is usually where you would place you initial filtration media (like filter pads, etc.) the water will then go into the other chambers and eventually, when the tank is nearly full, into the return chamber (the one with the pipe back into the tank). The pump then pumps the water back into the tank setting up a recirculating system. It is actually quite easy but a little different from HOB or over the back type systems. Look for information online for AIOs and you will get everything you need.
The hole is for an external sump. Your tank uses an internal sump. Look in the AIO build threads on R2R and you will see lots of information on the type of tank you have, in fact my tank is an AIO. If you click on my build thread badge, a lot of my earlier posts show the workings of an AIO section although it isn't designed to teach, at least you can see how one is set up.
