Beijing to Shanghai - Does Pre Booking Guarantee Me A Seat On My Selected Train

I am wanting to book the ride from Beijing to Shanghai on 7th October 2015. We are 2 adults and 3 children and need to catch a connecting flight in Shanghai. I notice when pre booking that it asks me for my back up option if my selected seat class or ride is not available. Does pre booking on this website not guarantee me a seat on my selected one?