The nipple is for using an external pump to drain the tank, It's not a vent. It's #1 in the pic.

 

Thanks! Now I know what it is (1 - Auxiliary capped fitting). I called it a vent because I didn't know want else to call it. But since it comes with a rubber cap over it, it isn't going to vent anything. I figured it was for some other application, but the pumps were more of a universal type so if not needed, it got capped.