Yes jacob kevin is refering to the outside half of the overflow and the inside half of the overflow. The reason the outside half is deeper than the inside half is to enhance the syphon of the U tube.
Grant,
An overflow box is used as jacob said when you need to get water that is in the tank out of the tank. Overflows are usually assocated with wet/dry filter or sumps. If you have a canister filter or a hang on filter you dont need or want an overflow.
Some tanks are "reef ready" and they have overflows built into the tank it self. These tanks are also sometimes refered to as "drilled" tanks. An add on overflow is something that hangs on the back of the tank. There are two box's that make the overflow up. The inside oveflow box has slits cut in to the top to allow water to flow into it. The outside box has atleast 1 drain hole in it aswell as its usually split up into two sections. 1 section is where the drain hole is and the other section is where a "U" tube is placed. The U tube by the action of a syphon will pull water out of the inside box and put it into the outside box. The water will then flow over the division and into the drain section of the outside box. It will then flow down to the sump or wet/dry filter. A pump will then return the water back up into the tank.
In theory the sump pump is always overfilling the tank and the overflow is what allows the tank to drain in a controlled manor back into the sump.
If you size you return pump larger than the overflow can process then you have a problem as the tank WILL overfill. That is why knowing the headheight pump capacity and the overflow capacity is important.