Alternate access settings provide a mechanism for server farm administrators to identify the different ways that portal sites are accessed, ensuring that URLs are displayed appropriately for the access method. For more information, see About Alternate Portal Access Settings.
You cannot delete the access setting for a portal site that is in use. You must delete the portal site and then delete the access setting. For information about deleting a portal site, see Deleting a Portal Site. You also cannot delete the access setting for a document library (Web Storage System-based) that is in use. you must delete the document library and then delete the access setting. For information about deleting a document library, see Deleting a Document Library.