A physical thing like an industrial robot is represented as a digital twin.
Sometimes this thing contains one or more subsystems that are complicated enough to warrant their own digital twins. There’s a parent/child relationship between the subsystem digital twins and the digital twin of the the overall physical thing.
Sometimes the the physical thing belongs to a group of other physical things. For instance, a collection of industrial robots working together forms an assembly line. That group could also be described as a digital twin. There would be a parent/child relationship between the member digital twins (industrial robots) and the digital twin of the overall group (assembly lines).
As you might imagine, a collection of groups could also be represented as a digital twin. For instance, a collection of assembly lines forms a factory. Another example would be how a collection of conference rooms, offices, and floors would form a commercial office building.
The properties of child digital twins map to the properties of the parent digital twin to express causal relationships.