The Invoice Recurring Transaction Summary form will be opened when recurring invoice transactions. This form will list all the Invoices created after processing a recurring transaction for all the invoices selected, thus giving you summary on the newly created invoices.
In the PO Number field, the only entry field, enter the PO Number specific to the Invoices created. There is also an ellipse button to the left of the Invoice ID that when clicked will bring you to the actual transaction.
See Override Transaction Numbers of Recurring Transactions to show you how to use the Invoice Recurring Transaction Summary form.