Answer: State postulate states that,
A simple compressible system is completely specified by two independent, intensive properties
This holds true for single-phase systems where only two intensive properties are enough to define the state of the system. But in the Rankine cycle during heat addition and heat rejection, the phase of the system changes. Hence to define the state third property pressure is introduced in the T-s diagram. These pressure lines are constant during phase change but they vary during sensible heat addition. When these saturation points at different pressures are joined they form a dome shape.