Hello Xonizai41
When using group tickets, you will benefit from discounts because you allow the railway operators to be prepared for you as a group; for example, it allows them to plan the rolling stock. Since these are discounted tickets, the train operators ask you to make a mandatory seat reservation on IC and IR trains; it is also possible that the railway operator will refuse your itinerary due to lack of space on the desired services, e.g. due to other groups.
You may change your travel plans at short notice; however, keep in mind that you could face a denied boarding due to lack of space or that there are no seats left for you.
Also, the group must travel together; each group must be accompanied by a group leader.
Kind regards,
Roland