About Assembling a Layout
You can assemble a layout (*.cem) file in an assembly. A layout, similar to a skeleton is the first component in the assembly. When you assemble a layout directly into an assembly, the entire layout is assembled, you cannot filter for selected geometry. The layout is assembled by default to the assembly csys. You cannot redefine layout placement. A layout component in the assembly is always up-to-date with the original layout.
The layout geometry can be scenery in the assembly, used for referencing in component placement, and creating assembly features.