This read-write TruthValue enables or disables Quality Level (QL) mode for synchronization status messaging (SSM) on the OCn/STM interface, and per its description, when enabled, the receive side processes SSM slow-protocol PDUs and the transmit side periodically transmits them, applicable only to synchronous OCn/STM interfaces. An administrator enables it when this interface is used as a timing reference source or distribution path that must exchange clock-quality information with adjacent nodes. For example, enabling QL mode on an OCn interface feeding a network's primary reference clock lets downstream nodes automatically learn and react to that clock source's quality level via SSM.