How you add a transaction from an electronic statement to the register depends on whether it affects a transaction that's already in QuickBooks or not.
It affects an existing transaction: If the transaction is a payment from a customer, you may need to mark an invoice as paid. If it is a payment to a vendor, you may need to mark a bill as paid. Or, if it is a deposit of customer payments that you received in QuickBooks, you need to record the deposit.
It does not affect anything: The transaction to enter is not related to any bill, invoice, or received payment in QuickBooks.