Transmission fluid change

Car listed above is an LX model with 127,000 miles and a 2.2L engine. I acquired this car and want to change the fluid in the 5 speed manual transmission. It would seem that there should only be two plugs to remove: 1) the drain plug located just below where the passenger side axel enters the transaxle case and which is removed with a 3/8" ratchet and 2) a 17mm filler plug located just to the left of the axel. However, there is another plug on the transaxle case which is also removed with a 3/8" ratchet. This plug is located on a part of the transaxle case that is closer to the wheel and is to the right of the axel as you look at it. It is at about the same height as the 17mm filler plug. What is the purpose of this plug?