@Brucanna means that supplier credits cannot possibly be associated with customers.
Whatever you actually mean, you should read these Guides before asking further questions. They will probably answer whatever questions you have:
https://forum.manager.io/t/credit-notes-for-customer-returns-and-refunds/7425
https://forum.manager.io/t/debit-notes-for-supplier-returns-and-refunds/7426