This button keeps annoyingly lighting up when I expand the track view of an instrument. I have no idea what it does or the consequences of having it lit or unlit but it is a bind to have to click it a couple of times (every time) to put it out. Does anyone know if it has any real function that I should be aware of and why it keeps lighting up?
 
many thanks

When it is lit, input is echoed through the synth. IOW, you can hear the result of pressing keys on your MIDI controller. It could be lit because the "Always Echo Current MIDI Track" is enabled in Preferences > MIDI > Playback and Recording. I prefer to manually control input echo, some don't that is why there is preference setting.