Breadwinner will pull in Credit Notes (sometimes called Credit Memos) from your accounting system, but for security reasons, Breadwinner will not create Credit Notes.
So any Credit Notes created or edited in your accounting system will be visible in Salesforce, but you can't create them in Salesforce/Breadwinner.
If you attribute a Credit Note to an Invoice in your accounting system, Breadwinner will be aware of that allocation and will adjust the Invoice accordingly.
The Credit Note will be associated with a Salesforce Account, but we will not automatically associate it with an Opportunity (though you can do this yourself manually). This is because Credit Notes can be related with no Invoice, one Invoice, or even multiple Invoices.
We occasionally get requests for Breadwinner to create Credit Notes. However, most finance teams consider it a positive 'feature' that Breadwinner does not create Credit Notes, as it allows them to have control over this process.
Breadwinner will read/replicate Credit Notes back to Salesforce/Breadwinner, but it will not create or edit Credit Notes.