The sensitivity function is a way to identify whether a wire is an input or an output or a power supply. If it is a power supply, the sensitivity is zero to everything within the circuit If the wire is an input, the sensitivity is unity to the voltage at that point. If that voltage is held fixed, the sensitivity to everything else in the circuit is zero. If the wire is an output, the sensitivity is fixed with respect to the input, and zero with respect to everything else, if the input is held constant.