 Two-letter region subtags are based on codes assigned, or "exceptionally reserved", in ISO 3166-1.If the ISO 3166 Maintenance Agency were to reassign a code that had previously been assigned to a different country, the existing BCP 47 subtag corresponding to that code would retain its meaning, and a new region subtag based on UN M.49 would be registered for the new country.UN M.49 is also the source for numeric region subtags for geographical regions, such as codice_1 for South America.