Have you ever purchased a ticket for yourself and someone else? Then the other person was not able to travel?
Tickets are non-transferable and non-refundable. So if the person that the ticket was purchased for does not travel, they can only be used by that person. The seat would not be yours to use.
I had a party of 3 the other day, 1 of the 3 did not travel. The 2 thought they would be able to have the 3rd seat empty between them. That is not how that works. If there is an unoccupied seat it will be given to another traveler. The ticket purchased that was not used will lose the purchased amount of the ticket.
If you advise the airline in advance that the person is not traveling, then they can use the amount in the ticket for travel at a later date, within 1 year of purchase. Only for the name on the ticket, no one else can use the amount.
If you want to have an empty seat, you can do that, It must be booked under your name, as an extra seat. You must tell us when you board the flight.
Enjoy the Journey
IFR