Full-float and semi-float usually pertain only to solid axle designs. Fully floating means that the wheel hub/bearing is separate from the axle shaft itself, and attached directly to the axle housing. None of the rig's weight is handled by the axle shaft, just the torque from the differential.
Semi-floating means that the wheel mounts directly to the axle shaft, and the shaft rides on a bearing. The shaft is usually either retained by a C-clip at the differential, or a bolted retainer at the outside of the wheel bearing (like the 9"). Heavy duty applications like 3/4 and 1-ton trucks usually get full-floating axles due to their better weight-handling capabilities. Semi-floaters that use a C-clip have the nasty habit of letting the wheel walk away when an axle shaft breaks, whereas ones with a pressed-on bearing and retainer plate will at least keep things mostly together as you coast to the side of the trail/road/track.
1994 Super Coupe ('93 5.0L swap), 1990 Tbird 3.8L, 1982 Honda CB900F, 1972 F-100 SportCustom 4x4, 1970 Chevy Custom Camper / 20