Looks like it the ID of the heating element. Taken from the GE website: "The heating elements on some GE and Hotpoint Appliance ranges are called "Calrod" heating units. The Calrod™ heating unit is manufactured by threading a coiled wire through a metal (steel alloy) sheath. Magnesium Oxide is poured around the wire to keep it centered in the sheath. The ends of the element are sealed with a ceramic material. The sheath is then formed to fit the oven cavity. If the Calrod™ is to be used as a surface unit, the side of the sheath is flattened and the element is formed into the familiar surface unit coil. This flattened side provides a flush contact with the bottom of a cooking vessel, thus transferring heat into the vessel efficiently."
