Correlation Meter gauges the phase relationship of a stereo signal.
Correlation Meter scale values indicate the following:
A +1 correlation value indicates that the left and right channels correlate 100%. In other words, the left and right signals are in phase and are the same shape.
Correlation values to the right of the center position indicate that the stereo signal is mono compatible.
The middle position indicates the highest allowable amount of left/right divergence, which is often audible as an extremely wide stereo effect.
When the Correlation Meter moves to the left of the center position, out-of-phase material is present. This leads to phase cancelations if the stereo signal is combined into a mono signal.