Some routes and train operators require that you have a seat reservation with your ticket. If you don't see an option for adding seat reservation while booking, seat reservations are not possible. We always recommend adding a seat reservation if possible, to not risk having to travel standing up. That's not very cozy.
With most tickets comes an opportunity to choose where on the train you'd prefer to sit, where the two most common options are Window and Aisle. Unfortunately, we can't promise that you are able to get a seat according to preference, as it depends entirely on how many free seats there are left on the train. We hope to be able to present a more comprehensive way of choosing seats in the near future, with options such as quiet section and animals allowed.