Sinking and Sourcing inputs simply refer to the current flow in a transistor. This means that they require a voltage and a load to operate. A sinking input requires the voltage and load to be present before connecting it to the circuit. This means that it is "sinking" the current to ground for the circuit. A sourcing input must be before the load in the circuit. This means that it is "sourcing" the current to the circuit. Voltage and a load must be present in either case to detect a voltage change at the input. The same is true for sinking or sourcing outputs.