A credit is when the bank is disbursing funds. It is easy for banks to do an international direct deposit account for credits. The bank knows the credit history of the customer. It has the data to decide if the account should be credited immediately. Or, the bank may decide to allow the customer limited use of the funds. Or wait to disburse until after the FX settles.
A debit is when the bank collects funds. Processing debits is more challenging than processing credits. Debit transactions require that the bank determine the credit worthiness of both its own customer and the person on the other end of the transaction. The bank has several choices
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