Drawdown is the average reservoir pressure minus this pressure. The productivity index divides the rate by that drawdown. The inflow performance relationship plots rate against this pressure.
It is measured with a downhole gauge or estimated from the tubing-head pressure and the weight of the flowing column. The estimate is reliable for a single-phase liquid and much less reliable once gas has broken out and the mixture density depends on slip between the phases. A well test that reports only a tubing-head pressure has not reported this number until that conversion is made.