Yes! And sometimes, no. It's mandatory on some rail services, optional on others and not available at all on the rest. Reserve and choose which type of seat you would prefer by using the 'Preferences' screen in the app.
Unfortunately, you cannot choose a specific seat to reserve. You’ll only know which seat has been reserved for you after you’ve bought your ticket. You can find all your reservation details in ‘My tickets’ > ‘Booking details’.