Credit is issued to clients as a favor or in exchange for a task or service from a client. Credit can also be used for recording over-payments received on a previous invoice. Credits can then be applied to reduce the outstanding balance on an invoice in the form of a discount.
Write-offs are used when you know that a payment from a client is not going to be received. By writing off invoices, it reduces the outstanding balance on the invoices and applies to payments as an irrecoverable amount. Choosing to write-off invoices also keeps it from showing up on the aging reports.
Both credit and write-offs, when applied in CORE, automatically create a general journal entry to adjust the tax liability for Accounts Receivable (A/R) invoices. The relevant tax reports for both are also adjusted accordingly. Check CORE Help Center for details on Payments.