always require an explicit response (confirmation or refusal) to a cancellation

3.3 Cancellation
A Print Buyer or a Print Provider sends a Cancellation to cancel a previously sent Business Object.PurchaseOrder. Note a Print Buyer will typically cancel a PurchaseOrder, if the Print Product is no longer required, whereas a Print Provider will typically cancel a PurchaseOrder if no useable artwork has arrived within a deadline.
For a Print Provider to cancel for a Quotation, it SHALL send a Cancellation whose @BusinessRefID references the
Quotation.
For a Print Buyer or a Print Provider to cancel for an RFQ or PurchaseOrder, it SHALL send to Print Provider a Cancellation whose
@BusinessRefID references the RFQ or PurchaseOrder.
If a Print Buyer sends a Cancellation, the Print Provider SHALL respond with a Confirmation or Refusal.
Note: If a Cancellation is accepted, production of the job will cease. Any work that has already been accrued can still be invoiced. If a Cancellation is refused, e.g. if the product has already been produced and shipped, it can still be delivered.
References: PurchaseOrder, Quotation, RFQ

3.10 PurchaseOrder
The Print Buyer accepts a Quote of the Quotation for purchase of a Print Product by creating and sending a PurchaseOrder that references the Quote. However, a PurchaseOrder can also be the first Business Object in a Negotiation Phase, especially for a reorder of a previously produced Print Product.
If a Print Provider accepts a PurchaseOrder, it SHOULD send a Confirmation that references the PurchaseOrder. If a Print Provider does not accept a PurchaseOrder, it SHALL send a Refusal that references the PurchaseOrder.