bath emergency drains?

whenever i take a bath, i notice that there is a circle made of metal under the spout, now i know that it is used to drain the tub if you leave the water on the water will the flow safely through the four rectangular holes on the side of the circle, here is a picture http://i.ehow.com/images/GlobalPhoto/Articles/4779746/116393-main_Full.jpg see it? its right under the faucet but above the drain. here is a diagram, http://www.us.kohler.com/onlinecatalog/550wide/ccc19695.jpg well i have a very old 80s bathtub, but it is the same design. well anyway printed/stamped into the metal is a sentance, "price pfister bath clean out" and i am just dieing to know what it means, so i hope the are some plumbers on yahoo answers, so what does it mean?