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

An «Adjustment Fee» value will be subtracted from the total credit memo (refund) amount.
An «Adjustment Fee» value should not be 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

When you create a credit memo for a partial order, the refund for the gift wrapping and/or gift option is included to the first credit memo created for the order. To exclude these costs from the amount to be refunded, the total of these costs should be specified in the Adjustment Fee field located above the Grand Total value in the credit memo.

http://merch.docs.magento.com/ee_docs/content/order-processing/adjustment-fee.html

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.

stackoverflow.com/a/21952861

See also:

• What is an «Adjustment Refund» for a credit memo?
• How to make a partial refund?
• 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
• How are the methods setAdjustmentPositive(), getAdjustmentPositive(), setAdjustmentNegative(), getAdjustmentNegative() of the \Magento\Sales\Model\Order\Creditmemo class used?
• How are the methods setBaseAdjustmentPositive(), getBaseAdjustmentPositive(), setBaseAdjustmentNegative(), getBaseAdjustmentNegative() of the \Magento\Sales\Model\Order\Creditmemo class used?
• How are the methods setAdjustmentPositive(), getAdjustmentPositive(), etAdjustmentNegative(), getAdjustmentNegative() of the \Magento\Sales\Model\Order class used?
• How are the methods setBaseAdjustmentPositive(), getBaseAdjustmentPositive(), setBaseAdjustmentNegative(), getBaseAdjustmentNegative() of the \Magento\Sales\Model\Order class used?