It can be helpful to think of Limber's limbs as having two components:
The underlying skeleton or geometry that provides animation features like separate lengths and FK blending.
The way that we set up the limb layers on top of that geometry, eg. with colors and shapes.
The rigged limb layer is kind of like putting flesh and clothing on the skeleton. In some other IK tools, there is no clothing until you add your own artwork - the tool only provides you with the skeleton. We wanted to also provide fully rigged limbs so that you can get animating in just a few clicks, without having to spend hours on a long preparation process.
So, although there are actually any number of ways of setting up layers to follow a geometric skeleton, we had to settle on something that we think works best for a wide range of situations. In the end, we came up with two very different but equally useful solutions. And these are the types of limb that Limber can generate when you click the New button: tapers and bones.
Taper limbs are based around three circles that you can change the size of. The middle circle is where the elbow or knee bends; so the joint always looks good, whatever angle you make it. Tapers can be split into different colors at two points along the length of the limb, and those splits also respond to rounding controls so that they can have a sense of volume and faux-3D perspective.
Bones are based on a super-simple path, with a stroke applied - they’re optimized to be extremely fast to preview and render. They're about as close as you can get to visualizing the skeleton of Limber's geometry. Bones can be altered by adjusting Stroke properties just like any other shape layer.