Racor fuel filter base - 3 lines?

[skiptowns98033]

I've got 3 lines going into the base of the fuel/water filter. I get that one is 'in' and the other is 'out' but what's the other one that has a female hose fitting?

 

[T.R. Bauer]

Mine has 2 outs for twin engine setups. I've never seen one with just one output - for single engine set up like my other boat, I just capped the extra.