I don't understand why S1 is equal to or greater than the higher of Ground Minimum Control Speed or Critical Engine Failure Speed (Vcef). I do understand that S1 has to be the lowest of Refusal Speed, Rotation Speed, and Max Braking Speed. Take a situation where Vcef is 100 knots, and the upper bound for S1 is 120 knots. It would seem that the only factors that should matter in determining S1 would be the three speeds that determine the upper bound. Why does the manual even talk about the lower bound if you are always taking the upper bound as S1.
What is so special about Vcef that we have to be above it? I would think that being below it is not a bad thing since it would take less room to stop than it would to lift off if you lost an engine.