A reference to another rigidbody this joint connects to.
If not set then the joint connects the object to a fixed point in world space.
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.