World models should be evaluated as a three-tier capability ladder rather than as one undifferentiated category: rendering (what the world looks like at the pixel/video level), simulation (the physical and dynamic reality — physics, interaction), and planning (predicting actions and future steps from the model). Each tier subsumes but is strictly deeper than the prior one.
It gives engineers a concrete evaluation axis that separates surface-level generative plausibility from genuine physical grounding. A system can score well on tier 1 (rendering) while failing tier 2 (simulation), so benchmarks and acceptance criteria must be defined per tier rather than aggregated into a single 'quality' score.
Any generative or predictive system that claims to model a domain can be audited by asking which tier it actually operates at: does it reproduce appearance, reproduce dynamics, or select actions? This applies to video models, robotics policies, and simulation environments alike.
Rendering shows what the world appears to be (pixels, videos).
Simulation captures the physical and dynamic reality of the world (physics, interaction).
Planning predicts actions and future steps based on the world model.
Video generation models rendering pixels vs. simulating how a water bottle moves when poured.