In addition to the QOS events that are sent between elements in the pipeline, there are also QOS messages posted on the pipeline bus to inform the application of QoS decisions. The QOS message contains the timestamps of when something was dropped along with the amount of dropped vs processed items. Elements must post a QOS message under these conditions:
The element dropped a buffer because of QoS reasons.
An element changes its processing strategy because of QoS reasons (quality). This could include a decoder that decides to drop every B frame to increase its processing speed or an effect element switching to a lower quality algorithm.