After receiving a purchase order, the supplier uses the 855 to tell the buyer what it can actually deliver. The header (the BAK segment) references the original PO number and states whether the order is accepted, accepted with changes or rejected. At the line level, ACK segments can confirm or change the quantity, price or scheduled ship date for each item, so the buyer knows what to expect before anything ships.
The 855 is not the same as the functional acknowledgment (EDI 997). A 997 only confirms that a file arrived and could be read; the 855 is a business answer about the order itself. It usually follows the 997 and comes before the advance ship notice. If the buyer then revises the order with a PO change, the supplier replies with a PO change acknowledgment.
Buyers rely on 855s to plan inventory, receiving and their own customer promises. A late or missing 855 leaves the buyer guessing; an 855 that says “accepted” when the supplier will short-ship is worse. Many retailers set deadlines for 855s in their compliance requirements, track acknowledgment timeliness and accuracy on supplier scorecards, and issue chargebacks for repeated misses.