Simplify the following text for a Grade 9 native English-speaking person.
Original: The ISO 3166-1 alpha-3 codes are used most prominently in ISO\/IEC 7501-1 for machine-readable passports , as standardized by the International Civil Aviation Organization , with a number of additional codes for special passports ; some of these codes are currently reserved and not used at the present stage in ISO 3166-1 . The United Nations uses a combination of ISO 3166-1 alpha-2 and alpha-3 codes , along with codes that pre-date the creation of ISO 3166 , for international vehicle registration codes , which are codes used to identify the issuing country of a vehicle registration plate ; some of these codes are currently indeterminately reserved in ISO 3166-1 .
Simplified: Reserved code elements are codes which , while not ISO 3166-1 codes , are in use for some applications in conjunction with the ISO 3166 codes .