Emergency one. Normal lane departure assistance will only vibrate the steering wheel. Enough people have complained and hopefully Tesla will allow us to permanently disable it or totally remove it.
As of 16.x, there are multiple options.
Normal lane departure has warning and assist modes. In warning mode, the wheel vibrates, in assist mode it will nudge the car back into the lane.
Emergency lane departure is a separate option to toggle, which is supposed to react if the car is drifting lanes and it thinks there may be a collision.