The main stereo outputs come from the stereo bus (via the stereo fader) or the mix/master file; they do not have their own metering bars. The monitor outputs come from whatever you select via the Monitor Select button, and whatever is being monitored will show on the right-hand meter bars, so you can use it as a 'probe' to check the sound & levels at various points within the machine, including the stereo output. It can do all of this without affecting anything else.