These channels will not be exposed as JACK output ports when enabling the Create per-instrument outputs option in the Preferences.
: mutes the instrument.
Fader: the fader next to the component's name allows you to adjust the volume of all layers (of all instruments) associated with this component. In the background of the fader a volume unit (VU) meter is included representing the instrument's signal level. The displays shows its peak value.
You can adjust the falloff speed of the peak values in the display in the Interface tab of the Appearance tab in the Preferences.