Do not call this directly (which is why it is called "internal"). Instead, call predictPairProb. Heuristics implement this function in order to be callable with predictPairProb.
predictProbInternal(object, row1, row2)
The object that implements predictPair, e.g. a ttb model.
The first row of cues (object$cols_to_fit columns), as a one-row matrix.
The second row of cues.
A value from 0 to 1, representing the probability that row1's criterion is greater than row2's criterion.
Most heuristics have not implemented this. Also, the output cannot (and should not) be assessed with categorical measures like percent correct.