According to the FCOM:
Green dot speed.
Engine out operating speed in clean configuration.
(Best lift to drag ratio speed).
Corresponds also to the final takeoff speed.
Represented by a green dot on the PFD scale. Below 20000 feet equal to 2 × weight (tonnes) + 85
Above 20000 feet add 1 knot per 1000 feet
when is it normally used ?
In approach mode, with speed managed and the aircraft clean, this is the target speed. It is also the target holding speed. In reality, if minimum clean speed is required, a speed a few knots above green dot is selected (i.e. speed is no longer managed).