On the saturation line, pressure alone fixes everything: name the pressure and the table hands you the temperature and every enthalpy. Above the line that stops being true. Superheated steam at 2,000 kPa can be at 300 °C or at 500 °C, and its enthalpy is different at each. So the superheated table has two coordinates, and we write $h = h(p,\,T)$, read aloud h equals h of p and T. $h$ is the specific enthalpy in kJ/kg, $p$ the absolute pressure in kPa, and $T$ the steam temperature in °C. Pressure picks the block. Temperature picks the entry inside it.

There is no short formula behind those entries. The printed table is generated from the IAPWS-IF97 industrial standard, a long polynomial fit that nobody evaluates by hand. Your job is to read it correctly, and then to subtract. Enthalpy has an arbitrary zero, so a single value means little. Every use of this table ends in a difference.

The first difference is the heat added per kilogram between two states at one pressure: $q = h_2 - h_1$, q equals h-two minus h-one, where $h_1$ is the inlet enthalpy, $h_2$ the outlet enthalpy and $q$ the heat added, all in kJ/kg. Scale it by the steam rate and it is a duty: $\dot Q = \dot m\,(h_2 - h_1)$, Q-dot equals m-dot times h-two minus h-one, with $\dot m$ in kg/s and $\dot Q$ in kW. The subscripts follow the steam: 1 is where it enters the section, 2 is where it leaves.

Two nuggets. At the same temperature, steam at a HIGHER pressure carries LESS enthalpy: 3,052 kJ/kg at 1,000 kPa and 300 °C, but 2,962 at 4,000 kPa. Read the wrong block and the error is silent. And the same 50-degree rise costs about 106 kJ/kg in one corner of the table and 131 in another, so a constant specific heat is a guess. The table exists because that number moves.