To define a URL mapping for a resource outside of SharePoint, you must supply a unique name, initial URL, and select a zone ...

To define a URL mapping for a resource outside of SharePoint, you must supply a unique name, initial URL, and select a zone for that initial URL.  The URL for this resource must be unique in the farm.  The name you supply will be the identifier for this set of mappings, and will be added to the list of items to select from via the Change button on the other URL mapping pages.  The name you supply must be unique within that list.