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 the perfusion is reversed.
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 :  minor.
  0.20  -> -0.20 :  undetermined.