The action "none" for Custom URL category was created because once the category was created, it could not be deleted. It has always had an action associated with it because PAN-OS viewed that as "in use". If the user wants to delete a custom category, they should set the action to "none" and then proceed to delete the custom URL category. If custom URL category action is set to "none" that is the same as if this configuration does not exist, Palo Alto Networks will use the predefined URL categories for traffic matching.
Note: If the user has a custom URL category and they do not want it to match as part of a URL filtering profile, they can also use the "none" action for that purpose.