For example, shellac is soluble in alcohol, meaning that if you add solid shellac flakes to the solvent alcohol, the shellac dissolves and you get a purely liquid solution of shellac and alcohol. Permeability is a measure of how porous something is. Wax may not be soluble in alcohol, but it is permeable to alcohol. Much as water can soak through a cotton shirt, alcohol can diffuse through wax. Solubility can come into play when a new coating is applied.
Solvent strippers take the effects of solubility to their logical, disastrous extreme.
If old layers aren't soluble, they may be permeable. The new coating or cleaning product can penetrate the permeable previous layer, whether it's wax, dirt, or degraded finish. When it passes through the intervening layers, it can dissolve into a soluble layer further down. In short, the solubility and permeability of the thin layers of coating, grime, and wax that make up an old surface can play havoc with an orderly layer structure. Because the layers tell a qualitative story of the sequence of events that has occurred to produce the old surface, and that story can be read and interpreted.