对此关节连接到的其他刚体的引用。
如果未设置，则关节将对象连接到世界空间中的固定点。
Unity does not support connecting a joint to a Rigidbody in a different Scene that is using a local physics Scene. If a joint is connected to a Rigidbody, and then that Rigidbody is moved to a Scene that uses LocalPhysicsMode.Physics3D, then the joint is automatically disconnected from the Rigidbody.
See Also: Scene and LocalPhysicsMode.