Depletion of the high voltage battery means exactly what you think it does - it's running out of fuel to the point that the car stops. It does not mean *complete* depletion of the anti-bricking protection, it means when the car stops due to running out of charge, it's not covered. In the past, Tesla has extended a one-time goodwill gesture in hauling you to the nearest charging point, but it's never been guaranteed. In general, you run out of charge, you'll pay for the tow.