The US dollar actually has three codes assigned: USD, USS (same day) and USN (next day). According to UN/CEFACT recommendation 9, paragraphs 8-9 (ECE/TRADE/203, 1996, available online:
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
In applications where there is a need to distinguish between different types of currencies, or where funds are required as in the banking environment, or where there is no field identifier, the third (rightmost) character of the alphabetic code is an indicator, preferably mnemonic, derived from the name of the major currency unit or fund – example: USD for general, unspecified purposes; USN for United States dollar next-day funds, and USS for funds which are immediately available for Federal Reserve transfer, withdrawal in cash or transfer in like funds (same-day funds). Since there is no need for such a distinction in international trade applications, the funds codes have not been included in the Annex to the present Recommendation.
