In electronic circuit drawings, there are two types of objects: component symbols and nets. Nets represent wires connecting the components – which represent physical devices. In the example below, we see component type MAX3232. U2 is a reference label of a component. Electronic component usually has pins. Pins always have their numbers starting from 1. Pins also have their names. They are usually written inside component blocks like C1+. For this particular component, we used the U2 label. U (or IC) label applies to all semiconductors. But you know that resistors usually are labeled as R1, R3. Capacitors C1, C2, Diodes – D1, D2, Transistors Q1, Q2, Crystals – X1, X2, X3, Jumpers J1, J2, J3, Inductors L1, L2.