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ISO 3166-1 に存在しない国名を表すために ZZ などを使って良いとのこと。

"User-assigned codes - If users need code elements to represent country names not included in ISO 3166-1, the series of letters AA, QM to QZ, XA to XZ, and ZZ, and the series AAA to AAZ, QMA to QZZ, XAA to XZZ, and ZZA to ZZZ respectively, and the series of numbers 900 to 999 are available."

ISO - Glossary for ISO 3166 https://www.iso.org/glossary-for-iso-3166.html