def sub_size_tag_from_sub_size(sub_size):
    """Generate a sub-grid tag, to customize phase names based on the sub-grid size used.

    This changes the phase name 'phase_name' as follows:

    sub_size = None -> phase_name
    sub_size = 1 -> phase_name_sub_size_2
    sub_size = 4 -> phase_name_sub_size_4
    """
    return '__sub_' + str(sub_size)
