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 disease.
The certainty factors are in this case being translated to the following expressions:

  0.80  ->  1.00 :  is.
  0.40  ->  0.80 :  probably is.
  0.20  ->  0.40 :  possibly is.
 -0.20  ->  0.20 :  equivocally is.