The 'outstanding' column for invoices will display the amount outstanding in the invoiced currency. For the customer row, the 'outstanding' column will display the total exposure in your Xero base currency (the converted sum of all outstanding invoices).
If there has been a part payment against the invoice, the outstanding amount will be underlined. This indicates that you may hover over the outstanding amount with your cursor to see the total invoiced amount.
All currencies for customer rows are exchanged using the current rate from openexchangerates.org.