C for coil or control. Not sure. A relay is a mechanical switch controlled by an electromagnet composed of a coil of wire wrapped around an iron core, as depic

C for coil or control. Not sure. A relay is a mechanical switch controlled by an electromagnet composed of a coil of wire wrapped around an iron core, as depicted on the icon. When current flows through the coil, it pulls the metal spring towards it, which can either allow current to flow from "in" to the output or disconnect the current flow from "in" to the output. Those are the two types of relay components on offer.