In Unity Entities, a SubScene is a separate authoring unit that holds GameObjects and MonoBehaviours while you work in the Editor, then bakes that hierarchy into entity and component data for runtime. The SubScene is not a classic gameplay scene you load with SceneManager in the old sense. It is a conversion boundary. Content inside it goes through the conversion pipeline so the runtime world ends up with ECS data (entities, buffers, shared components, and so on) instead of a full GameObject tree.