"To identify the most discriminating concepts, a reiterative procedure analogous to stepwise regression was performed. In the first iteration, the group classification was performed using only one concept at a time, determining which single concept of the 30 resulted in the highest classification accuracy. In the second iteration, the classification was performed using pairs of concepts, namely the single concept that produced the highest accuracy in the first iteration as well as each of the 29 other concepts. All pairs that produced at least as high an accuracy as achieved on the previous iteration, were explored in the third iteration, where triplets of concepts were used, namely the pairs that produced the highest accuracy in the previous iteration, plus each of the remaining 28 concepts. Such stepwise addition of discriminating concepts continued until adding any one of the remaining concepts resulted in a decrease in accuracy. An analogous procedure identified the most discriminating locations."
But I still think even in your case they are doing:
train: abc; val: d -> score1/ features0 -> features1
train: abd; val: c -> score2/ features1 -> features2
...etc