Is partial cancellation reflected to a PO?

The shippingCount you can find when you query PO, is the initial purchase quantity where returned quantity is not reflected. 

You can find returned quantity from cancelCount.  

• shippingCount = Purchase quantity of an order
• holdCountForCancel = Canceled and to-be-refunded quantity
• cancelCount = Quantity for which cancellation is confirmed.
• Orderable count = shippingCount - (holdCountForCancel + cancelCount)