A semi float axle pulls double duty- it carries the weight of the vehicle, and it transfer the power from the ring and pinion to the wheels. A semi float axle can only be changed if the wheels are removed because the wheels are bolted directly to the axle flange (more or less). Because the axle carries the weight of the vehicle and transfers torque at the same time, it becomes easier to damage the axle under high torque loads with a heavy vehicle.
A full float axle has only one duty - to transfer the torque from the ring and pinion. The vehicle weight rests on a seperate flange that is part of the axle tube that the wheels are bolted to. A full float axle shaft can be removed without the vehicle jacked up or the wheels removed. Simply unbolt the flange and pull it out. Because the only function of the axle shaft is to transfer torque, it is less likely to suffer damage. The actual weight of the vehicle rests on the axle tube (more or less).
When I first started getting into 4 wheeling, I would go to the junkyard just to look around and take stuff apart. I wouldn't buy anything, I would just see something on a truck that I wouldn't recognize, and try to take it apart to try to understand it. Full floaters were one of those things.