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 CavityRatio. CavityWidth is the radial distance , expressed in disk pressure scale heights , over which the surface density and viscosity transitions occur. More precisely, log(viscosity) is an affine function of between and , where is specified by [CavityRadius->22]. Note that the surface density is only approximately initialized to the corresponding cavity, so the initial state is not a steady state. If you want a steady state, you should allow your system to relax over a few viscous timescales.