ジョイントの端にアタッチされた Rigidboy2D オブジェクト（すなわち Joint コンポーネントのないオブジェクト）
もしこのプロパティーに null を設定するとジョイントは別の Rigidbody2D オブジェクトでなく空間上の固定地点にアタッチされます。
Unity does not support connecting a joint to a Rigidbody2D in a different Scene that is using a local physics Scene. If a joint is connected to a Rigidbody2D, and then that Rigidbody2D is moved to a Scene that uses LocalPhysicsMode.Physics2D, then the joint is automatically disconnected from the Rigidbody2D.
Additional resources: Scene, LocalPhysicsMode, Rigidbody2D and collideConnected.