def upload_global_tag(task):
    """
    Get the global tag that is supposed to be used for uploads for the given task.

    Parameters:
      task (str): An identifier of the task. Supported values are 'master', 'validation', 'online', 'prompt', data', 'mc', 'analysis'

    Returns:
      The name of the GT for uploads or None if a new GT should be created by the client for each upload request.
    """

    if task == 'master':
        return None
    elif task == 'validation':
        return None
    elif task == 'online':
        return None
    elif task == 'prompt':
        return None
    elif task == 'data':
        return None
    elif task == 'mc':
        return None
    elif task == 'analysis':
        return None