During training, if an instance from q i is correctly classified by the learner, it will be "promoted" to q i+1 (solid/green arrows in Figure 1), otherwise it will be "demoted" to the previous queue, q i−1 (dashed/red arrows in Figure 1).
as the learner trains through time, higher queues will accumulate instances that are easier for the learner, while lower queues will accumulate harder instances.