If CircuitWorks cannot find a suitable component model in its library, then it uses the ECAD data to build a component model and adds it to the Component Library for future use.
ECAD files contain simple descriptions of component shapes that CircuitWorks uses to build component models, such as this transistor:
In some situations, such as when using the SOLIDWORKS assembly for thermal analysis, these simple component models are preferable. If more realistic models are required, you can replace models in the library with more detailed models that you create from SOLIDWORKS: