Sliding Scale Tariff is also known as Average Power Factor Tariff.
In this type of tariff, an average power factor is set as a reference power factor. If a consumer has his power factor below this reference point, additional charges are made. However, if power factor of consumer is above this reference, he is provided with some discount.
Power factor is defined as ratio of true to apparent power.
Also see here Power factor correction
See MCQs related to Power Factor here: