Why it is represented in terms of U T S that is surface current? Because surface currents are quite easy to measure so you, when you want to design a structure. For example, even you want to design you just design for the whole, first what you do, you design you evaluate the force distribution along your obstruction or the pressure distribution. And then you look at the total force and the moment, in a similar way you need to have the distribution of the currents. So, when you want to have the distribution of currents, one simple method which we have in hand is the one-seventh power formula, which has been discussed here.