The point is that the number of features reduces by one at each node during training, but the features could themselves be scores from a probability distributio

The point is that the number of features reduces by one at each node during training, but the features could themselves be scores from a probability distribution. It would be a form of preprocessing. The threshold is selected to minimize the difference in size of the split (training) sets, or to maximize information gain. During classification, each node has a binary output.