Orders rarely stay the same from creation to shipment. Demand moves, promotions shift and delivery plans change. Instead of canceling and reissuing a purchase order, the buyer sends an 860 that references the original PO number. The header (the BCH segment) identifies the order and the type of change, and each changed line (a POC segment) says what is being added, deleted or modified, such as a new quantity, price or delivery date.
The 860 follows the 850 and, usually, the supplier’s PO acknowledgment. The supplier answers with a PO change acknowledgment (EDI 865), accepting or rejecting the changes. Shipments and invoices must then reflect the revised order, so the advance ship notice and invoice are reconciled against the latest version of the order, not the first one.
Changes are a common source of mismatches. If the supplier’s system doesn’t apply the 860, or applies it after picking has started, the shipment matches an outdated order. Some trading partners don’t support 860s at all and expect changes by email or phone, which leaves no reliable record. A full change history for each order is what lets both sides settle disputes about what was actually agreed.