A distinguishing characteristic of fluids is that they cannot carry tensile forces (i.e. pulling, sucking, dragging...) and can rather
only produce compressive and shear forces.
So if an airfoil is regarded as
70% of its lift on the upper surface and around 30% from the lower surface
, I read this as: Of the total wing loading (N/sq.meter or psf), 70% of that is reduced atmospheric pressure on the upper surface, and 30% is increased pressure on the lower surface.
