Sinking and sourcing are terms used to define how direct current flows in a load from a PLC output. Sinking provides a grounded connection and sourcing provides a voltage source. Sinking uses a common ground line while sourcing uses a common positive voltage line. This allows multiple inputs to connect to a single common return line, reducing the number of terminals needed on the PLC. Sinking and sourcing refer to whether the output provides ground or voltage to the load in a circuit.