It's a condensate duct to ensure that moisture doesn't run straight back down into the fan/room. Bit of a major design though, much sleeker and easier setups than that to deal with the condensate
I suspect that the 'bulge' highlighted below represents the fan and it's shown offset to stop any rain that comes down the vent entering it. But that's just my interpretation of the drawing, without more details, I'm only guessing.....
......Then on second thoughts, if it is the fan, what's missing is the drain off for any collected water.
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