Credit Notes are records of credit given to a customer within Xero e.g Refunded amounts, gift cards.
For a more detailed explanation from Xero, click here.
ProntoXero syncs the record of the Credit Note from Xero to Salesforce and shows credit amounts on the related Account, however you cannot create a Credit Note from within Salesforce.
There will be a related list on the Account in Salesforce that shows the Xero Credit Notes with their Total Amount and Remaining Credit.
The Account also has fields for Total Invoiced, Total Paid, Total Outstanding, and Total Available Credit.
Applications of Credit Notes in Xero will show on the Xero Invoice in Salesforce under the Xero Credit Note Allocations related list, and there are fields on the Invoice itself which show Amount Credited and Amount Due.