Comment by chutsu

"Assembly" as in if you have multiple parts in Fusion360 etc, you can select components and put them together by forming constraints. E.g. a screw into a mount hole with a nut on the other side. Expensive CAD software makes this relatively easy, you just select the objects and define the constraints. This is lacking in CAD software like FreeCAD (not very well supported), TinkerCad (where you have to create throw away objects), OpenScad is equally not easy, you have to manually position the objects and line them up in code to "assemble" them.