thermodynamics - Joule-Kelvin coefficent what does $p$ and $T$ represent?

In the Joule-Kelvin expansion we take a gas at one pressure $p_1$ and throttle it through a valve to another pressure $p_2$ where these two pressures are taken to be constant. The Joule-Kelvin coefficient is given by:

$$\mu_{JK} = \left(\frac{\partial T}{\partial p}\right)_H$$ So what temperature does $T$ represent and what pressure does $p$ represent?