Power Target % - The target wattage over max TDP (represented as a percentage) to feed the GPU. Power Target was introduced with Maxwell overclocking on NVIDIA devices as a means to increase power consumption over the lower TDP base, thus allowing further headroom for overvolting and overclocking. A higher watt draw gives more room for more power consumption by the GPU, which gives more room for overclocking. If users attempt to overclock at the stock 100% TDP on, for example, a reference GTX 980, they will eventually run into limitations with the 180W power draw; increasing Power Target to 125% (180 * 1.25 = 225W), that gives an additional 45W to increase clocks and voltage.
Overvolting should be done as rarely as possible when overclocking Maxwell, as it will eat into the power budget shared by the clocks and the voltages. Read more below.