PHX (physics sub-assembly).
Robots and complex objects can be created using .mphx or .mworld files. These files can be imported directly into a world or PHX using a reference to the file. For example, a humanoid robot can be broken down as follows:
Next, build the Robot.mphx, that links 2 legs with 1 body, 1 head, and 2 arms using joints. Robot.mphx can be imported directly into the world using several reference like /robot1, /robot2 that link to robot.mphx.
Use joints to make links between PHX bodies. If a static joint is used, it is optimized and deleted automatically by the engine if the 'Auto optimize' property is checked. In this case, all the geometries referenced in body2 are moved to the body1.