There are scenarios in which extra credits will be created, such as you dropping a subject or two. These credit notes may be used to contra with your Invoice, to be refunded, or transferred to your Credit Wallet so it could be used in a future registration.
An email will be sent to you if the credit has been refunded, a receipt if it was used as payment for any invoices, or been transferred to your Credit Wallet.
What is a Credit Wallet? Please refer to: Credit Wallet