If you are driving trucks with Z plates (test drives, trips to and from technical truck inspection,…) you don’t always know which type of truck you will be driving on a certain day.
Therefore you need to register your OBU at the highest weight class and the lowest emission class of the trucks you will be driving with that Z license plate. Mind that this can be the values of 2 different trucks. You need to scan the vehicle documents of those 2 corresponding vehicles to prove the emission and weight class when registering your Z license plates and OBU.
Please bear in mind that you are at all times responsible for the values that you have registered and that you risk a fine if you use the Z license plates and OBU on vehicles with a higher weight or lower Euro class than actually registered on the OBU.