bitsPortQLModeEnabled is a read-write boolean that turns Quality Level (QL) mode on or off for this BITS port; when enabled, the receive side peers incoming SSM slow-protocol PDUs and acts on them, and the transmit side sends out its own SSM PDUs. An administrator enables this when the port participates in SSM-based automatic clock-source selection, and disables it on ports where the quality level must be assumed rather than signaled. For example, disabling QL mode on a BITS-IN port is what makes bitsPortAssumedQL meaningful, since without SSM signaling the device must be told the incoming reference's quality level instead of learning it.