You are driving a vehicle with a gross vehicle weight rating of 7 t at 60 km/h outside of a built-up area. When do you have to maintain such a great distance, that an overtaking vehicle can pull back into lane?

You answered correctly. When you drive a vehicle over 3.5 tons outside of built-up areas and there is only one lane in your direction, you must keep a large enough distance from the vehicle in front so that another vehicle can safely move back into the lane after overtaking. This rule ensures safe overtaking maneuvers since slower, heavier vehicles can form bottlenecks and must allow enough space for overtaking vehicles to return without forcing them to cut in dangerously. This rule does not apply when there is more than one lane in your direction or if overtaking is not allowed, because safe returning is either not needed or overtaking is forbidden. Remember this for similar questions: the key is the combination of vehicle weight, location (outside towns), and the number of lanes.