If you make a new limb 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, 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.
Limber is smart, so it can duplicate, as well as copy and paste custom limbs, but you need to keep a version of them in an After Effects project file somewhere if you want to be able to re-use them in the future.
The Limb Library included in the Limber download is such an After Effects project file that contains a selection of custom limbs that we’ve designed for you. Unlike other IK tools, these are not just style presets that you could make yourself. They're sophisticated rigs in their own right which add alternative, dynamic functionalities to the default limbs.