What is Assembly?
Assembly is collection of Icepak Objects (e.g. PCB, Grills, Fans etc.) that are grouped and stored as a single unit.
Creating Assembly helps in organizing the existing model, and it can also be exported (write assemblies) to be used in different models.
Typically objects which are close to each other are put in an assembly.
Assembly can be defined as a non-conformal Assembly using "Mesh separately" option in Assemblies ----> Meshing Panel.
Using non-conformal assembly helps in reducing the mesh count in a model by confining the mesh within that particular assembly.
For additional information, refer following section:
Icepak 12.1 User Guide -> Building a Model -> Custom Assemblies
Also, check out attachment for more information.