If you make a new taper or bone by clicking the New button, you've made what we call a default limb. Default limbs are the rigs that Limber has stored in the .jsxbin script file, ready to create at any time.
If you were to go and alter something in the limb layer, like changing a bone to a different color, or adding your own artwork to a taper, that limb is no longer a default limb. It's now what we call a custom limb. Custom limbs are not permanently stored by the script, like default limbs are.
The Limb Library included in the Limber download contains a selection of custom limbs that we’ve designed for you, which add advanced styling options that the default limbs don’t have. Most of the limbs in the library are based on bones or tapers, whilst some are more unique and can't really be said to belong to either type.