As soon as a Purchase Order (PO) is confirmed as split receipted, a new PO will be created for the remaining product quantities that have not been receipted from the original. The new PO will have the same order number as it's original but it will include a numerical suffix e.g. /1 to differentiate it from the original and it will be created in a Placed status. 

If any of outstanding stock on the new Placed PO is no longer expected to be received, you can delete the individual product lines using the bin icon on the PO's order lines or you can delete the PO using the red Delete button in the top right of the PO's page. Deleting the order lines from the Placed PO or deleting the new PO as a whole will not impact the products or stock that have already been receipted.

For more details, see How to Split Receipt a Purchase Order.