Thanks for the reply. However, according to the battery manufacturer, it is a "drainage" line for battery acid, in the (hopefully unlikely) event that the charging system malfunctions and overcharges the battery. In that case, the acid would drain, directed downward through the hose, rather than spewing out into some undesirable direction under the hood. According to the manufactruer, the battery has vents, but that's not what the hose is. This satisfies my intuition, as a footlong hose that hangs downward wouldn't be a good "vent". Does this sound correct?
In any case, whatever it is for, Interstate Battery has a warehouse that is only a few miles from my house, and they have the adaptor kit, which will allow me to hook up my OEM hose (round cross-section) to the ovular port on the battery. So, I plan to pick this up and install it, in the next couple of days. Thanks.
2002 C230K: Brilliant Silver Metallic, Charcoal Leather, Auto, C5 Premium Pkg (Panorama + Rain Sensor + Bose), CD Changer