This feature will handle your callback when an invoice receives a payment after the invoice expired.

It doesn't mean that your invoice can be paid even if the invoice expired. In the ideal condition, invoice that has expired can't be opened, thus your end customer will not be able to pay. 

This feature will help to notify your application of the recent status of the callback, if your customer pay the invoice before expiry but there is a late callback from partner while the invoice has expired. 

Please be informed that, late callback from partner is a rare case. However, we can suggest you to tick "Also notify my application when an invoice has expired" to avoid a discrepancy of callback status.