Represented by the top of an amber strip along the airspeed scale on the PFD.
Computed by the FAC, based on aerodynamic data, and corresponds to 1.13 VS during
takeoff, or after a touch and go.
Becomes 1.23 VS, after retraction of one step of flaps.
Becomes 1.28 VS, when in clean configuration.
Note: If in CONF 0 VLS were 1.23 VS (instead of 1.28 VS), the alpha protection strip
would hit the VLS strip on the PFD.
Above 20 000 ft, VLS is corrected for Mach effect to maintain a buffet margin of 0.2 g.
In addition, VLS increases with speed brakes extension.
F : Minimum speed at which the flaps may be retracted at takeoff.
In approach, used as a target speed when the aircraft is in CONF 2 or CONF 3.
Represented by “F” on the PFD speed scale.
Equal to about 1.18 VS to 1.22 VS of CONF 1+ F.
S : Minimum speed at which the slats may be retracted at takeoff.
In approach, used as a target speed when the aircraft is in CONF 1.
Represented by “S” on the PFD airspeed scale.
Equal to about 1.22 VS to 1.25 VS of clean configuration.
O : Green dot speed.
Engine-out operating speed in clean configuration.
(Best lift-to-drag ratio speed).
Also corresponds to the final takeoff speed.
Represented by a green dot on the PFD scale.
Below 20 000 ft equal to 2 × weight (tons) +85
Above 20 000 ft, add 1 kt per 1 000 ft
The above is from Airbus. The reason you can use auto thrust and select and fly down to VLS is that is may be needed to non normal approaches (flap or slat problem etc). Or you may be heavy weight in some types (345/6, 380) and need to select below S or F speed to establish below Vfe.