Here are two examples to think about pressure on a shell model.
Imagine a steel drum where there is a screw-in plug to seal pressure in the drum. With the plug removed, there is equal pressure on the inside and outside of the drum so the applied pressure would be zero and there is no deformation. Now boil some water in the drum and when it is filled with steam, screw in the plug and float the drum in a tub of ice cold water. As the steam condenses and the temperature goes down inside the drum, the pressure on the inside of the drum will go down, but the atmospheric pressue on the outside of the drum doesn’t change. At some point the drum collapses when the pressure difference is larger than the buckling strength of the drum.
If you take another drum and screw the plug in at sea level when the pressure on the inside of the drum is 101,000 Pa. Then take the drum up to an altitude of 9km, but keep it at the same temperature as it was at sea level the air pressure drops to 31,000 Pa. If you have a shell model of the drum, you would apply an outward pressure of 70,000 Pa. The sides of the drum would extend outward.
It is always the pressure difference. If the wing is not pressurized, the static pressure on the inside of the wing is Pinfinity, the same as the air pressure at whatever altitude the plane is flying at. The pressure in the air moving over the outside of the wing surface is either higher or lower than Pinfinity. It is the difference that is applied to the shell model surface.