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

You answered correctly. The numbers "156/150" in the tyre designation indicate the load index for single and twin tyre use. The first number, 156, is the load index for a single tyre, and the second number, 150, is for when the tyres are mounted in twin formation (dual tyres on one axle). The load index tells you the maximum weight the tyre can safely carry in each mounting situation. The other options refer to tyre dimensions or properties, not to load index. Knowing this allows you to choose tyres suitable for your vehicle's weight requirements, especially important for trucks and buses.