A Shock Level measures the high-tech concepts you can contemplate without being impressed, frightened, blindly enthusiastic - without exhibiting future shock. Shock Level Zero or SL0, for example, is modern technology and the modern-day world, SL1 is virtual reality or an ecommerce-based economy, SL2 is interstellar travel, medical immortality or genetic engineering, SL3 is nanotech or human-equivalent AI, and SL4 is the Singularity.
The classification is useful because it helps measure what your audience is ready for; for example, going two Shock Levels higher will cause people to be shocked, but being seriously frightened takes three Shock Levels. Obviously this is just a loose rule of thumb! Also, I find that I often want to refer to groups by shock level; for example, "This argument works best between SL1 and SL2". (This does not mean that people with different Shock Levels are necessarily divided into opposing social factions. It's not an "Us and Them" thing.)