Data type for currency 'type' column, e.g. usd, cat, etc.
3 letter ISO currency codes are relatively small, so there is little overhead in using them as the key for the reference table.  Having certain items such as dates, GL codes and other analysis codes and currency codes directly on the tables is quite convenient for people reporting off the transactions and balances.  Many accounting packages - even big players like Oracle Financials - do it for this reason.