Three connection oil bath

Ok. I posted this before, but still curious. Thought I would try again, now that I have a pic of someone else's setup.

I have an oil bath unit with three connections. One connection for the air horn. I know that. The connection to the left is for the dipstick connection. I know that too. The third connection is a little tube that I have currently plugged on the right side of the air horn connector. Where does that hook up to and what was it used for? Sample pic shows a line coming off of it to the left side of the engine.