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The state ID can indicate which country a repeater belongs to. In the United States, Canada and Mexico. The state ID also indicates which state or province the code represents. Everywhere else, the state ID represents just the country the repeater resides in.
Repeaterbook is not using a FIPS code to specify the country of the United States. This is a historical issue as Repeaterbook was initially designed to only display repeaters in the United States. The state ID represents the state FIPS code, which is a two-digit numeric character.
Canada and Mexico state IDs include the country designator along with the state or province FIPS code. Their codes are a four digit alphanumeric character. The first two characters are the country's FIPS code (Canada is CA and Mexico is MX). The last two characters are numeric and represent the province (Canada) or state (Mexico) the repeater resides in.
All other countries use a two-digit alpha character to represent the country that can be found here.