Volume normalization in different scenarios

Someone can point me to the documentation on how Volume Normalization works? I thought it was to apply the replaygain or r128, or?
When it is active, where can I see which settings are applied?
For example, in Roon it’s very easy to see it in the track info:

How does it work with Spotify Volume Normalization?
@balbuze How does it work when the DRC plugin enabled? Is it adding attenuation to the DRC process?

Volume normalisation only works for mpd. So Spotify is not affected by this setting

OK, so only mpd.
How does it work? R128? REPLAYGAIN?
How can you check what is doing?

On the Spotify side? Does the plugin uses the Spotify volume levelling?

@balbuze is this Enable replaygain permanently using custom shell script (volumio-mpdcorr.sh) the only way to enable replaygain?
How does this differs from volume normalization?
Is there a way to check how volumio is applying those settings?

@and1 thank you so much for your script and for your answers. I’m still unclear on what ‘volume normalization’ does. I’m reading from the mpd documentation “If yes, mpd will normalize the volume of songs as they play.”, but I cannot find what is using

thank you!