PT is a structure (gridded output) or array (scatter output) containing pressure-temperature points (MPa and Kelvin). 'material' defines which ice, water, or solution to use. Possibilities: 'Ih' for ice Ih (Feistel and Wagner, 2006) ...
The pressure in the air determines the precise temp at which the water can freeze. At higher altitudes where the pressure of the air decreases, the water will freeze at an elevated temperature than at sea levels. This is why you observe ice formation on mountain peaks before it forms in va...