def extract_clas(the_sidd):
    """
    Extract the classification string from a SIDD as appropriate for NITF Security
    tags CLAS attribute.

    Parameters
    ----------
    the_sidd : SIDDType|SIDDType1

    Returns
    -------
    str
    """

    class_str = the_sidd.ProductCreation.Classification.classification

    if class_str is None or class_str == '':
        return 'U'
    else:
        return class_str[:1]