On FlixTrain you can book a seat separately if you want a specific seat. Otherwise you get an assigned seat included with the ticket for no extra charge.
I.e. unlike DB there are no overcrowded FlixTrains where people have to sit on floors or stand.
FlixTrains have a maximum occupancy dictated by number of seats.
And includes a reserved seat.
I went for 4.99€ last month – twice (taking early morning trains).