From Dayco:

Today's belts are now made with an EPDM extended-life rubber compound.  These EPDM constructed belts wear completely different than Neoprene.  Dayco's new aWEARness Gauge is designed to gauge wear and estimate belt wear.  By simply placing the notched area lengthwise over the ribbed section of the belt, this pocket size device quickly and easily identifies the potential of serpentine belt failure.  If there are more than four cracks per inch in the belt, trade practice recommends the belt be replace.