Closing Invoices work in the same way that 'Staged payments' do in the sense they are also created from the "Payment Plan" using the Create Invoice(s) option in the Lines FastTab. This is only marked as Ready to Invoice once all the 'Staged Payments' have been posted.
The purpose of the 'Closing Invoice' is to effectively invoice anything that hasn't been included in the 'Staged Payments'. For example, you may have set up your 'Staged Payments' to cover 80% of the total "Sales Order" value. In which case, the "Closing Invoice" will generate for the remaining 20%. If your 'Staged Payments' cover the full Total Incl. VAT on the "Sales Order", the 'Closing Invoice' is still required, but this will generate at Zero value.
As the process to create the 'Closing Invoice' uses the Get Shipment Lines functionality within Business Central, this will only ever invoice based on what has been shipped.
When the 'Closing Invoice' has been created, the final document will look a bit like this:
This is made up 'Comment' Lines which will detail the Shipment No (or numbers where multiple shipments posted) as well as your Item lines pulled from your "Sales Order" to show what was included on your original order.
Another 'Comment' line is then used as a header to show the 'Payment Plan Invoices' followed by 'G/L Account' lines for your 'Staged Payments', (not including any that have been reversed). In the same way that the Unit Price is calculated on the 'Staged Payment' Invoices, the Unit Price Excl VAT is a %/fixed amount calculation based on how your 'Staged Payments' are setup, using the Total Inc VAT value to assign the correct value per invoice.
As mentioned above, anything that was not invoiced in your 'Staged Payments' is leftover and will be invoiced within this 'Closing Invoice'.
The Factbox on the "Sales Order" will also show the total Invoiced Amount, which, once all the 'Staged Payments' and 'Closing Invoice' have been posted, should equal the Payment Plan Amount.