Hi Praneeth,
Once order is TECO, you cannot do confirmations GI and GR with respect to that order number. All the existing reservation number and capacities gets deleted after order is techncially closed(TECO). But in standard system allows to perform unplanned issues against the order even after TECO. Ie; you can book the cost on order even though it is TECO till order is closed. Once order is closed you cannot book any cost.
Regards,
Ramagiri