The details of currency handling are controlled by settings in the [InvoiceCurrency] section of Eiglobal.ini and Eiglobalextra.ini.
You can adjust:
Which currencies INVOICES is able to look for on invoices.
The currency notations that INVOICES looks for by default, both when looking for the Currency field and when you specify currency notation for amount fields.
The three-letter abbreviation that INVOICES (1) assigns to the Currency field when a corresponding currency notation is found, (2) uses in several dialogs, and (3) includes in the output.