This object reports the effective, currently-in-force quality level of a specific candidate input clock source, computed from whichever of three factors takes precedence: the most recently received QL, any locally forced or configured QL, or an override triggered by a fault condition like signal failure. It is the number the G.781 selection algorithm actually uses to rank this source against its competitors when deciding which one becomes T0, so it directly explains why one source was picked over another. If two sources both physically report up but this effective QL differs between them, that difference, not simple availability, is what determines which one the device prefers.