Others can correct me if terminology is wrong, but a double acting valve is usually spring return to center. Cylinder extends or retracts as long as you're pushing lever one direction or another. When lever is released it returns to center position, cylinder is "locked" in last position.
Float valves operates same as double acting valve, except you can push lever beyond its normal operating range to an additional detent position where it stays until manually returned to the neutral center (closed) position. In detent position the cylinder will float. That is, unpressurized hydraulic oil is free to move in and out of the cylinder and the cylinder offers no mechanical resistance. When I use rear blade to plow snow in winter I put the blade's "tilt" cylinder in float so that the blade can follow contours instead of one edge digging in or raising above snow. This is similar to having float on a 3 point hitch tilt cylinder so that a backblade can follow the grounds contour instead of rotating with every bump and dip of tractor's rear axle.
"Float detent" is just one type of detent valve. There's also detent valve that when the lever is moved beyond the normal spring return range to detent position the valve will stay open and pressurized hydraulic flow will be continuous. The detent position on this valve is used for applications like a hydraulic pump or a wood-splitter that require continuous flow.
