Computer Aided Design (CAD) 3D Assembly Design
Putting all the 3D part design content together in a 3D assembly 
Since most parts do not 'stand alone', but are integrated into sub and top level assemblies to achieve the desired function. 3D assembly design is integrated with 3D part design (when required) since the assembly will always influence the 3D part design - you usually can't have one without the other! Assemblies can get very complex, quite quickly. The interactions between the different parts needs to be well understood at an early stage, not only from a design point of view, but also manufacturing and purchasing. This enables a better designed assembly from a hierachial point of view - this helps with the creation of accurate sub-level assemblies which aid manufacturing and purchasing, especially if Product Data Managament (PDM) and procurement systems are used.