This parameter is specific to a set of simulations performed to evaluate the efficiency of planet trapping at a disk temperature transition. Unless you want to reproduce these simulations, you should simply ignore this variable. If you use it, you also have to define TransitionRatio and TransitionWidth. TransitionRadius is the approximate radius at which the transition occurs. This set of three variables has to be used much in the same spirit as the CavityRadius, CavityRatio and CavityWidth set of variables (which describe a surface density transition, rather than a temperature transition).

Note that TransitionRadius can be varied in time using the LambdaDoubling parameter.