Yeah, I guess process of elimination would make it the one light of those three that you can't identify.
It comes on when the main tank is empty, and the reserve tank is all that's left. It will flicker a bit as the fuel is around the trigger point for it as the fuel sloshes and activates it and then sloshes again and de-activates it.
It will light for 2 seconds as you turn the ignition on to serve as a bulb-check for the rider.
The sensor is pretty solid, but has been known to fail occasionally. That might also cause your flickering.
2010 Honda NT700V
1986 Shadow 700
1986 Honda Trail 110 (Postie Bike)