Simplify the following text for a Grade 9 native English-speaking person.
Original: The country codes have been published as ISO 3166-1 since 1997 , when ISO 3166 was expanded into three parts to include codes for subdivisions and former countries .
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 .