The 855 document is sent to GovX to confirm not only that the order was received, but whether it will be fulfilled or rejected.
For example, in the ideal scenario, an 855 can be used acknowledged the entire order and communicate that all the items will be fulfilled. The 855 can also be used to communicate an order rejection either partial or full. In the case of a partial rejections, only some of the line-items or perhaps, some of the qty of a particular item will be fulfilled and the remainder rejected.
See the attached 855 mapping specifications for details on acknowledgement code.