Railjet, Intercity, Eurocity and Euronight trains have dedicated parking spaces for bicycles. By making a reservation, you are guaranteed a parking space for your bicycle. If the spaces on a long-distance train are already fully booked, you will not be offered a ticket. This means that overbooking is impossible.
You are welcome to take your bike with you on regional trains (Regionalzug), regional express trains (Regionalexpress) and commuter S-Bahn trains if there are enough free spaces. However, you cannot reserve a space for your bike on these trains.