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Following is explained in the ISO20022 Payments Maintenance 2009 document:
Definition: Code allocated to a currency, by a maintenance agency, under an international identification scheme as described in the latest edition of the international standard ISO 4217 "Codes for the representation of currencies and funds". Valid currency codes are registered with the ISO 4217 Maintenance Agency, and consist of three contiguous letters.
XMLdation validator checks that country is valid from an external list: http://www.currency-iso.org/isocy/gl.../table-a1.html