When you see this road sign, what is the situation in which you must keep a minimum distance of 70 metres from the vehicle in front?

You answered correctly. This road sign indicates that drivers of lorries with a permissible total weight over 3.5 tons, as well as tractor units, must keep at least 70 meters distance from the vehicle in front if that vehicle is another lorry over 3.5 tons or a combination that counts as a lorry. This rule is to prevent dangerous situations like pile-ups, especially on motorways. However, this distance rule does not apply when the vehicle in front is a car with a trailer. So, your selections match the German traffic regulation for this sign. Remember: the rule is specifically for heavy trucks or truck combinations, not for cars with trailers.