Hierarchical administrative subdivision codes
They are described in the book Administrative Subdivisions of Countries: A Comprehensive World Reference, 1900 Through 1998, written by Gwillim Law († August 31, 2016).
The codes are alphabetic and have constant length for the first level subdivisions. Examples:
DE - Germany DE.NW - North Rhine-Westphalia DE.NW.CE - Kreis Coesfeld