May be referring to the common 3 layer architectures (see Fowler's PoEAA) which map closely to ECS: Top layer is for "frontend", whatever that means for the pr

May be referring to the common 3 layer architectures (see Fowler's PoEAA) which map closely to ECS: Top layer is for "frontend", whatever that means for the product (UI, sound, simulation, etc.), the stuff with side effects. "Systems". Middle layer is purely functional, for business/domain logic AKA utility functions. The most liquid layer, but should not be confused as trivial. Bottom layer is where state (or a way to access & modify it) lives. Data access layer, component layer, etc.