I did a bit of digging, and on the Elexon glossary it says: “A Physical Notification is the best estimate of the level of generation or demand that a participant in the BM expects a BM Unit to export or import, respectively, in a Settlement Period. Initial Physical Notifications (IPNs), which are submitted to the System Operator at the day ahead stage, are not displayed on the BMRS or used in Settlement calculations. Final Physical Notifications (FPNs) have to be submitted to the System Operator by Gate Closure for each Settlement Period, and are both displayed on the BMRS and used in Settlement calculations.”

On the BMRS website, you can see the Final Physical Notifications on https://www.bmreports.com/bmrs/?q=balancing/physicaldata.