Hello Siukaileung
Thank you for your query.
The Super Saver ticket is only valid on the booked train and on the booked day, that is correct. A seat reservation is not necessary. There are also trains where you cannot buy a reservation. If you are asked by the train staff to disembark, please let your ticket confirm so that you can travel with the next possible connection.
I wish you a nice day.
Kind regards,
FatimaH