Table 2.

Status of research on DT application systems

Hierarchy of application system	Application areas of the system	Reference	Function of the application system
Unit-level DT	Aerospace	[43, 66–68]	Life prediction Damage prediction
	Bridge construction	[69, 70]	Building structural health monitoring Life prediction
	Transportation	[71–73]	Virtual testing Vehicle health monitoring
	Healthcare	[74, 75]	Surgical or medical simulation
Process-level DT	Intelligent manufacturing	[12, 76–82]	Production process simulation Diagnosis of machine faults Monitoring and forecasting of production tools or machines
	Human-machine collaboration	[55, 83–87]	Optimization of industrial human–machine collaboration
	Metal smelting	[88, 89]	Production process simulation
	Physical networks	[90–92]	Implementation of Industry 4.0, IoT and cyber-physical production systems
System-level DT	Smart factory	[93–97]	Improving the efficiency of shop floor manufacturing equipment Optimizing production planning and resource allocation Operational monitoring and troubleshooting
	Training and education	[98–101]	Optimizing distance learning or online teaching
	Energy and power	[58, 102–106]	Optimizing production planning and resource allocation Operational monitoring and troubleshooting
	Intelligent transportation	[107]	Increasing traffic condition awareness to aid driving decisions