On the TC 24 D and DA, the label on the lift lever is to indicate a range where your mmm will float with the NH cushion lift system. It is a system that takes most of the weight of the mmm off the gauge wheels, letting the tractor support the mower just enough so that the gauge wheels follow the ground, or "float" rather than dig in. Since it runs off the 3ph, they give an indicator for guidance. My 60" 914A mmm weighs almost 500 lbs, without the cushion lift, it would plow the gauge wheels into the ground on softer areas.
As far as true float goes on the 3ph, it is as others have said - there is no down pressure, so everything floats. It depends on the implement and design as to whether it is supposed to engage or dig in.
Hope this helps.
Brad