What are these for on my overflow?

I am trying to figure out what the 2nd grate is on my overflow. It appears to be a overflow for lower down but when i slide out the face plate it does not open up to anything. Does this grate do anything?

 

What do you mean it doesn't open up to anything??It is open to where the water goes into the overflow.It just allows the overflow a lower point in the tank to draw water.

 

if you look at the pictures there is nowhere for the water to flow from down below. Does it flow behind the face plate upwards towards the top overflow?

 

Yes.

 

As the water flows over the top of the overflow it pull the water from the grate the is in the middle of the overflow. The water flows upward from the grate that you pointed to up and over into the back of the overflow.

Makes sense?

 

Ah ok I understand now. Thanks for the help, I am new to the realm of overflows. I have been using canister filters, I'm building a new tank so i'm trying to understand everything before I plum it.

 

No problem that what we are here for