If you also select an existing limber layer (either before or after the other layers), Limber will intelligently apply the functions to that limb. If you don't select any limber layers, it will generate a new limb, based on the pose reference layers, and apply the functions to this new limb. If you are only posing, or only rigging artwork, you should still select layers in this order - just miss out the ones you don't need.