It depends on the ticket purchased.  The seats are automatically assigned upon booking and issuing the ticket.

For some ticket types you will be given the option whether you want a seat reserved, unless reservations are not available at all on the particular train you have chosen.  This could be because:

• The train services you are travelling on do not offer seat reservations eg if its a short journey.
• The train you wish to travel on is full at the time of booking.
• The train company may not have released seat reservations yet.  This is done up to 12 weeks ahead of travel.

Even without a seat reservation your tickets are still valid and you can sit in any vacant unreserved seat for the appropiate class of travel for your journey.