This parameter is specific to a set of simulations performed to evaluate the efficiency of planet trapping at a cavity edge. Unless you want to reproduce these simulations, you should simply ignore this variable. If you use it, you also have to define CavityRadius and CavityWidth. CavityRatio is the target surface density ratio of the outer to the inner disk. Outside of the cavity (r > CavityRadius), the surface density profile is whatever is given by Sigma0 and SigmaSlope, while inside the cavity it corresponds to this profile divided by CavityRatio. Similarly, outside the cavity the viscosity is whatever is prescribed by Viscosity or AlphaViscosity, while inside the cavity it corresponds to either value multiplied by CavityRatio.