Assemblies are collections of components that share design/manufacturing criteria. Typically assemblies map directly to the manufacturing tooling surface. For example, the panels on the upper wing skin (shown below) are built on the same tooling surface. These panels are assigned to an assembly because they are manufactured on the same tooling surface and share continuous plies for the skin and stiffeners.
Effective laminate blending and discrete laminate sequencing are performed on the assembly level (since assembly components share a tooling surface).