The main difference between Assembly Design and Digital Mockup (DMU) Kinematics in CATIA is that Assembly Design is used to create static assemblies, while DMU Kinematics is used to create dynamic assemblies. In Assembly Design, the parts are constrained together using geometric constraints, such as coincident, parallel, and perpendicular. This allows the assembly to be moved and rotated, but it does not allow the parts to move relative to each other. In DMU Kinematics, the parts are connected together using kinematic joints, such as revolute, prismatic, and cylindrical joints. These joints allow the parts to move relative to each other in a specific way.