AdjustmentAmount will indicate a debit or credit if any made by the seller towards the order amount.
When a buyer goes through checkout and pays for the order, AdjustmentAmount will contain 0. If the seller refunds the buyer, then the AdjustmentAmount will contain the refund amount as a negative number indicative of the credit given to the buyer by the seller. If the field contains a positive amount it indicates that the amount is an extra charge being paid to the seller by the buyer.
If the seller inputs a number in the SM Pro record for the order under "Seller discounts (-) or charges (+): ", then the number entered by the seller shows up in the AdjustmentAmount as is.