Yes that's correct. The port that is down is the one that allows air to flow into the barrel from the transfer port and valve in the back of the plenum.
Round one are for pellets, .22 and up also have a dual port on the opposite side of the barrels, that is more used for slug shooting, though as i understand it you can also shoot pellets there.
My.177 Maverick just have the single round port in the brass part in the end of the barrel, it is a 9 EURO upgrade to get the dual port option.