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 very specific simulations, you should simply ignore this variable. If you use it, you also have to define
TransitionRadius 
and
TransitionRatio 
.
TransitionWidth is the radial distance

, expressed in disk pressure scale heights

, over which the temperature transition occurs. More precisely, the logarithm of the disk aspect ratio

is an affine function of

between

and

.