Queues events selectively (either up or down). When a START_QUEUEING event is received from below, all subsequent events will be queued (in the up queue). The only events that will not be queued in this case is a STOP_QUEUEING: its argument is an event list that is passed up before the up queue is release. This means that all events in the up queue are passed up the queue, the up queue is emptied and queueing disabled.
The same scheme applies to down events. This layer is used by the STATE_TRANSFER layer.