The 812 explains money that changes outside the original invoice. A buyer may send a debit for a shortage, damaged goods, a pricing difference or a compliance violation; a supplier may send a credit for a return or a promotional allowance. Each adjustment references the related invoice and PO numbers and gives the amount, whether it is a credit or a debit, and a reason code describing why.
The 812 usually appears after the invoice (EDI 810) and the receipt have been compared. It often relates to a payment explained in a remittance advice (EDI 820), where the payer takes the adjustment. In retail, many debits come from vendor compliance programs: a late or inaccurate advance ship notice, a label error or a missed delivery window can each lead to a chargeback communicated this way.
For suppliers, 812s are an early warning. Each one is a deduction to check, accept or dispute, usually against a deadline. Vague reason codes, adjustments that can’t be traced to a specific shipment and backlogs of unreviewed debits are common, and deductions that go unchallenged come straight out of margin.