What is a 'Differential Lock' (Differenzialsperre) on a heavy truck drive axle, and when should it be engaged?

1. It locks the steering wheel in place while driving at 80 km/h
2. It locks both wheels on the driven axle together so they rotate at the exact same speed, providing maximum traction on slippery mud, snow, or loose terrain; it must be disengaged before turning on dry asphalt
3. It is engaged continuously on dry motorways to improve cornering
4. It is used only when refueling the fuel tank