M3 is a reference to the attenuation applied to a summed-mono signal, particularly in application to audio metering systems, to maintain electrical headroom and to give a perceived acoustic volume consistent with the stereo source. M3 uses a 3dB attenuation ((A+B) – 3dB ) and is most appropriate where the stereo source comprises largely decorrelated signals (ie. normal stereo).
M6 is a reference to the attenuation applied to a summed-mono signal to maintain electrical headroom and to give equivalent metering levels with the stereo source. M6 uses a 6dB attenuation ((A+B) – 6dB ) and is most appropriate where the stereo source has largely correlated signals (ie. dual-mono).