Technicly - and obviously, you are safe down to VLS. F or S indicates the speed at which you will be safe to raise the flaps or slats with no danger of VLS coming up to meet or even overtake your current speed. In most cases, VLS comes no where near F or S, but try a heavyweight departure out of a 6000' elevation airfield at ISA+20 retracting in the turn and in turbulence. One of our destinations is often like this - there's a fair amont of "Pucker Factor" involved as VLS shoots up to meet you!