Pressure-Saturation Coupling is a fundamental mechanism in finite difference reservoir simulation that describes the interdependent relationship between fluid pressure and phase saturation fields, wherein changes in pressure influence saturation distribution through Darcy-type fluxes and capillary effects, and, conversely, saturation-dependent relative permeability and capillary pressure functions modulate the pressure equation's coefficients, necessitating simultaneous or iterative solution strategies to ensure thermodynamic and numerical consistency.