What is an «Adjustment Refund» for a credit memo?

An «Adjustment Refund» value will be added to the total credit memo (refund) amount.
An «Adjustment Refund» value should be non-negative:

The value can be in percents (it should ends with the percent sign in this case):

Bug: the setAdjustmentPositive() and setAdjustmentNegative() methods of the \Magento\Sales\Model\Order\Creditmemo class wrongly use the \Magento\Sales\Model\Order’s getGrandTotal() method instead of the getBaseGrandTotal() when an adjusment is in percent, so it lead to a wrong calculation result

Take example your customer bought 2 chocolate each costing $5. However he/she wants to return one. In that case you have to return $5 which is correct mathematically (and this is what default magneto calculation shows you). But, you can change mind as business man, like

If you feel to giving return $6, write $1 in Adjustment Refund.
If you feel to give return $4, write $1 in Adjustment Fee.

Unable to generate Credit Memo in magento - Stack Overflow](Unable to generate Credit Memo in magento - Stack Overflow)

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