Fall protection is when you are attached by the dorsal 'D'. That is when a deceleration lanyard is used. Typically this is when in your in a bucket truck, or in other industries when a fall may happen. But there is usually footing to walk around or work, like bridge workers or something. If they fall of the bridge, the protection will catch them. The thing to keep in mind is a fall into fall protection hurts and it should be as a last resort. Not something that could happen at any moment. The deceleration lanyards and harnesses are only designed to fallen into once or not at all before they are replaced.
Climbing trees on the other hand, there isn't always that sure footing. That is where work positioning comes in. We use ropes and lanyards to give us the points of contact we need to work. In work positioning situations you should never be subjected to a larger fall. Our equipment is designed to catch us in a short fall of a foot or so, but not any more. It is there to keep us in one spot. The 3' above the tie in point thing is not a good idea (subjecting to a 6' fall). This is because we attach our ropes to our waist and if a fall did occur, our ropes to not have enough stretch to slow us down and internal damage can occur. Unlike fall protection, in work positioning, our body is not in line like it would be on a dorsal attachment,
I hope I did that justice and helped with your questions...