def METHOD_NAME(df: pd.DataFrame, df_add):
    """
    Method used to update the DataFrame on the category level.
    :param <pd.DataFrame> df: base DataFrame.
    :param <pd.DataFrame> df_add: appended DataFrame.
    """
    incumbent_categories = list(df['xcat'].unique())
    new_categories = list(df_add['xcat'].unique())
    # Union of both category columns from the two DataFrames.
    append_condition = set(incumbent_categories) | set(new_categories)
    intersect = list(set(incumbent_categories).intersection(set(new_categories)))
    if len(append_condition) == len(incumbent_categories + new_categories):
        df = pd.concat([df, df_add], axis=0,
                       ignore_index=True)
    # Shared categories plus any additional categories previously not defined in the base
    # DataFrame.
    else:
        df = df[~df['xcat'].isin(intersect)]
        df = pd.concat([df, df_add], axis=0,
                       ignore_index=True)
    return df