Quick summary- Straight Bottom most commonly used in a normal removal, possibly because the angle is easiest to cut from Up to Down because the kerf holds the weight of the saw. Straight Top, Angle from bottom to top commonly used for valuable saw timber, because it removes most of the notch wood from the root flare, which has no value at the mill. A more difficult notch because you have to remove more wood to clean the notch going through the (thicker) root flare and you have to hold the saw against gravity to make the cut. Angled top and bottom are used when you want the falling tree to stay on the hinge as long as possible, for instance when falling a leaner against or at angle to the lean. The wide notch lets the hinge bend as far as possible before it breaks, keeping the tree from jumping off the stump and providing better control of the direction of the fall. Again, more difficult than a standard notch because you have to remove more wood, against gravity, And you loose some of the benefit of the sighting line on your saw in "Aiming" your notch.
