It is possible to have any combination of currencies, multi-currencies, or single currency, including Salesforce and NetSuite not having the same currency structure. So it is possible to have NetSuite Multi-Currency and Salesforce as Single Currency.
Populating the currency of a Transaction requires being aware of the Currency ID, which can be found in the NetSuite Currency object. You might view a currency like USD, Salesforce might view that currency like USD, and NetSuite will view that currency as the currency with the ID of “1”.
With Breadwinner, you can either pass a three-letter text value which we will match to the ISO Code listed in the NetSuite Currency records in Salesforce, or you can directly pass an integer, which Breadwinner will pass to NetSuite directly (that integer will populate whatever currency NetSuite has as associated with that Currency ID).