When looking at the 3D phase diagram of water I felt unsure whether the only physical points were the ones on the surface of the plot.
If it is unphysical to move outside the surface of the phase diagram ("on shell"?) the question might be moot, but I guess there are conditions for when the phase diagram applies/does not apply. I am not sure when thermodynamics applies and does not apply; is perhaps heating ice by whispering at it too fast a process, or using a frying pan too fast a process, or microwaving too fast a process, or exploding with a nuke too fast a process, is it the energy flow per unit time over some volume that decides whether the process is too fast, etc.
So the question is when is a phase diagram valid and what kinds of processes follow the phase diagram and does the process necessarily need to lie embedded as a curve in the surface of the phase diagram?