Anything identifiable can be an entity
People, animals, products, vehicles, buildings, landscapes, clouds, fire, graphic shapes and text can carry identity. Open kinds such as living.person, object.vehicle.car and environment.landscape describe what an entity is without requiring an exhaustive enumeration.
Entities can have parts, relationships, appearance, geometry, presence intervals and multiple representations. A tree is an entity; a forest environment can contain many tree entities and still carry its own environmental properties.
A scene is a view of the world
A scene determines which representations appear and how they are arranged. Its coordinate system, camera, layout and layer composition belong to that view. An entity can appear in more than one scene without becoming a different entity.
A scene tree is appropriate for placement and parent-relative transforms. Relationship graphs capture other facts such as attached-to, looking-at, inside or following. Those facts should not be forced into a single containment tree.
Identity survives media changes
A product image and a product model can refer to the same entity. Switching between them should preserve reference relationships and semantic identity. A representation may be missing or unsupported without making the entity itself disappear from the specification.
Presence and lifecycle
A persistent entity can exist throughout an experience. An ephemeral entity can appear for a limited interval. Generated, procedural, tracked or conditional entities need enough provenance and conditions for a consumer to understand their presence.
Presence is not opacity. A fully transparent object can still exist, participate in relationships and require explicit treatment in interaction or semantics.