Breadwinner will pull in Credit Notes (sometimes called Credit Memos) from your accounting system, but for security reasons, Breadwinner does 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, in your accounting system, you allocate a Credit Note to an Invoice, then Breadwinner will be aware of that allocation and will adjust the Invoice accordingly.
While we will correctly associate the Credit Note with a Salesforce Account, we will not associate it with an Opportunity (though you can do this yourself manually). Why not? Because Credit Notes can be associated with no Invoice, one Invoice, or even multiple Invoices. As a result, we associate the Credit Note with a Salesforce Account but not an Opportunity.
We occasionally get requests for Breadwinner to create Credit Notes. However, most finance teams consider it a 'feature' that Breadwinner does not create Credit Notes. Finance teams prefer to have control over this process. That is why Breadwinner will read/replicate Credit Notes back to Salesforce/Breadwinner, but it will not create or edit Credit Notes.