The Mapping List displays default naming conventions for all skeleton objects. If the bones of your character have different naming conventions, use the Extract Naming Template button to create your own template of names. See Creating a bone-naming template.
A naming template is an .fbx file that contains the naming conventions you used on a skeleton. Later, you can use this template to map the bone objects of other skeletons that use your naming conventions automatically. See Applying a bone naming template.