The lengths of the limb determine how long it is, in pixels. From the center point of the hip to the center point of the knee is the Upper Length and similarly, from the knee to the ankle is the Lower Length.
The Start Size, Middle Size and End Size determine the diameter of the three circles of taper limbs, in pixels. Default bones do not respond to this property.
Bone Curvature applies a smooth curve to the joint of the bone. Default tapers do not respond to this property.
The Square End property cuts the end circle off, so you have the appearance of a squared-off end to the limb. Default bones do not respond to this property.
The Size Scale property will scale all the three circle sizes at once. If you have Link Length to Size Scale checked, the length of the limb will scale up proportionally, too. This enables you to scale a limb along with the rest of a character.
The Upper Split and Lower Split properties determine where the upper and lower parts of the limb are split, by a straight line, into different colors. The upper part uses the Upper Color and Middle Color, while the lower part uses the Middle Color and the Lower Color. The splits are a percentage of the length of each part, calculated from the centers of the controllers.
Default bones do not respond to these properties, but check out the Colored Bone in the limb library if you want a bone that does.
There are also Upper Rounding and Lower Rounding properties which will curve the straight line of the split into faux-3D perspective.
The Clockwise property will determine whether the limb's joint bends out to the left or to the right. Usually you’d set this to either 100% or -100% and then leave it, but it can be keyframed between these values for certain inbetweens. At 0% the limb will always appear perfectly straight regardless of where the controllers are, as if the joint was pointing directly towards or away from the viewer rather than out to one side.
The Rotate Start and Rotate End properties will enable the respective controllers to automatically rotate along with the limb, when they’re at 100%. At 0% the controllers will not rotate as they move around. You can keyframe between these values to make transitions between poses or cycles of animation easier.
The Stretch property enables your limb to stretch beyond it's nominal length. If unchecked, the limb will not extend to an end controller that is further away from a position where the limb is dead straight.
Anti-pop helps prevent the visual pops or jerking that can occur in IK animation, such as when limbs become straight too suddenly. It works by making the limb shorter as it approaches a 90º angle at the joint, and the normal length when dead straight or fully folded. Values above 50% tend to work best with Anti-pop. Anti-pop sometimes produces undesirable effects when you blend betwen IK and FK.
Anti-pop can smooth out pops and shivers in animation
Most of the time, you'll want to animate with IK. But some situations work better with FK, and you may find it easier to blend from one to the other than having to use a checkbox switch. This is when you would typically keyframe the FK property from 0 to 100.
You can then use Upper FK Rotation and Lower FK Rotation to control the angles of your limb, almost as if it were a pair of ordinary, parented layers. If you have a hand or foot layer parented to the end controller, you'll need to add an FK Controller, first.