1. If you estimated tax, Salesforce Billing provides a value for the Tax field and updates the Total Amount (With Tax) field for each of your credit note lines. If you click estimate tax again, Salesforce Billing performs the same series of actions and overrides your current tax fields with the new values.
2. If you applied tax, Salesforce Billing provides a value for the Tax field and updates the Total Amount (With Tax) field for each of your credit note lines. If your credit note is a draft, Salesforce Billing also posts it.