HI Shree
If you maintain planning time fence the order will be firmed as suggested by expert Tiago
when you run the planning heuristics the order related information will be not changed (ie charcterstics ) and the pegging relationship between requirement and receipt will be fixed .
It all depends on business scenarios to maintain planning time fence .in some mill cases they will keep short time because planned order must be trim later they as to convert production order .so finally planned order must be get deleted in planning run itself
Regards
Virender