What does "L" refer to in the following tyre designation: 315/80 R 22.5 156/150 L?

You answered correctly. The letter "L" in the tyre designation stands for the tyre's maximum permissible speed. Tyres are given a speed rating indicated by letters: "L" means the tyre is rated for speeds up to 120 km/h. This is important because using a tyre above its speed limit can be dangerous and is not allowed under German traffic law. Remember, the other numbers and codes on the tyre refer to load capacity and dimensions, but the speed rating is always shown as a letter at the end of the sequence.