Setting up sump, why is there a Bulkhead on the Side?

[dropdowntank]

Just purchased a used saltwater tank and sump, and wanted to convert to freshwater. I want ed to set up the plumbing and noticed that the compartment in the sump were I'm supposed to put the return pump there is a bulkhead on the side (towards the bottom) of the sump.
1) Why would a sump have that?
2) how can I utilize the sump to pump directly up to the tank? (without covering the hole)?
Thank You

 

[duanes]

Some pumps are made to sit out side the sump (cannot be submerged), but be plumbed in so only water runs thru the volute and impeller area.
These pumps vent heat to the atmosphere, not to the water.
Some pumps will sit on the bottom of the sump under water (submersible) and can pump out thru the bulkhead, where plumbing is structured to move water outside the wall, up to the tank.
I know people who put sumps in the basement, and tanks on the first floor to keep water movement, and pump noise outside the tanks room.
It also holds temp steady even at times when the main part of the house gets warm (too warm for certain fish or salt temp and reef stability.

 

[twentyleagues]

Yup what duanes said it's for an external pump. You can just cap it off should you not be using an external pump.