def positions_threshold_tag_from_positions_threshold(positions_threshold):
    """Generate a positions threshold tag, to customize phase names based on the threshold that positions are required     to trace within one another.

    This changes the phase name 'phase_name' as follows:

    positions_threshold = 1 -> phase_name
    positions_threshold = 2 -> phase_name_positions_threshold_2
    positions_threshold = 2 -> phase_name_positions_threshold_2
    """
    if positions_threshold is None:
        return ''
    return '__pos_{0:.2f}'.format(positions_threshold)
