Part 4/7:
A key aspect of successful simulations is incorporating "feedback" effects, where small-scale processes like star formation or supernovae have large-scale impacts on the system being modeled. Since computers can't resolve all these micro-details, modelers have to introduce "subgrid rules" - approximations and parameterizations that capture the essential effects.
This is similar to how weather and climate models incorporate rules for cloud formation that can't be derived directly from the underlying equations. It's a necessary compromise to make large-scale simulations tractable.