I think "u" stands for units. [tex]u^2[/tex] means units squared which is like saying ft^2 if you were looking for the area of something with feet dimensions. So in this case, the circle has a diameter of x units (u) and an area of x units^2 ([tex]u^2\\[/tex]). It is not the same as r. "r" is most always going to be used to represent the radius.
