Resend button appears when a failed order for which a recovery e-mail has already been sent is not recovered after a specific time (unrecovered time).
This time is set by the admin in “General Settings”.
For example, if it is set as “1 day” then if a failed order is not been recovered, then after 1 day of sending a recovery e-mail the resend button will appear and admin can re-send the recovery e-mail.