A Revit feature that packages a set of selected elements into a single unit with its own dedicated views and sheets, distinct from a Group's simple copy-and-place behavior. A precast concrete panel or a prefabricated bathroom pod is a natural fit for an Assembly - it needs its own detail views and a shop-drawing-style sheet for the fabricator, which is exactly what an Assembly generates automatically, unlike a plain Group which has no view/sheet infrastructure of its own.