Physics や (MonoBehaviour の FixedUpdate のような) 他の固定フレームレートの更新を実行するインターバル(秒)
Unity does not adjust fixedDeltaTime based on Time.timeScale. The fixedDeltaTime interval is always relative to the in-game time which Time.timeScale affects.
See Time and Frame Rate Management in the User Manual for more information about how this property relates to the other Time properties.