ISO 4217
8. In applications where monetary resources associated with a currency (i.e. funds) need not be specified and where a field identifier indicating currency is used, the first two (leftmost) characters are sufficient to identify a currency–example: US for United States dollars for general, unspecified purposes where a field identifier indicating currency is present. (A field identifier can be a preprinted field heading in an aligned document or a similarly-agreed application in electronic transmission of data.)