A stereo file has two ways of being wide, and only one of them survives being summed. This reads which one you have.
Phase correlation
Correlation compares the two channels sample by sample. +1 means they are identical, 0 means they are unrelated, and -1 means one is the exact inverse of the other.
The number is an average over the whole file, so a track that is mono in the verse and wide in the chorus lands somewhere between. The per-band figures and the scatter fill in what the average hides.
The goniometer
The scatter plots mid vertically and side horizontally, which puts the left and right channel axes on the diagonals. Four shapes are worth recognising:
- A vertical line: mono, or close to it.
- A rough vertical oval: a normal stereo mix.
- A circular cloud: very wide, and worth checking in mono.
- A horizontal line: the channels are inverted relative to each other. This is the one that disappears.
Points are sampled evenly across the whole file rather than taken from the opening seconds, so what you see describes the track and not its intro.
Mono loss
The figure under the verdict is the level the file loses when left and right are summed. A mono file loses nothing. A wide mix loses a fraction of a decibel. An out-of-phase pair loses everything.
Anything past about -3 dB is worth investigating before the file is delivered.
Correlation by band
The four bands are centred at 100 Hz, 400 Hz, 1.6 kHz, and 6.4 kHz. Bass should read close to +1; a wide top end is normal and mostly harmless.
If the low band reads low, the fix is usually to make the bass mono below a corner frequency rather than to narrow the whole mix.
Where it comes from
Correlation is the normalised covariance of the two channels, which is the same figure a hardware correlation meter shows. Mid and side are the sum and difference halved, the same pair the channel tool and the stereo width tool operate on.