PERFEX uses certainty factors to calculate the probability of everything it pronounces upon. In this case it is the certainty factor of whether or not a blackout area shows reversibility.
The certainty factors are in this case being translated to the following expressions:

  1.00  ->  0.80 :  major.
  0.80  ->  0.40 :  moderate.
  0.40  ->  0.20 :  some.
  0.20  -> -0.20 :  undetermined.