electrostatics - Is potential difference or potential used in defining capacitance?

In my textbook I came across the capacitance of a certain body (i.e. a sphere, not two different spheres as in a spherical capacitor) and in it the formula,

$$Q = CV$$

where $V$ is the potential of the body with respect to the Earth. Now in a parallel plate capacitor, why do we choose the potential difference and not the potential of a single plate to the Earth?

Answer

V in th either situation is potential difference but in the case of an isolated sphere as written in Halliday/Resnick ( Indian edition)

We can assign a capacitance to a single isolated spherical conductor of radius R by Assuming that the " missing plate " is a conducting sphere of infinite radius

So the potential on single sphere comes out to be potential difference