def message_optional_files_in_reports(calibration_file, cross_talk_file, head_pos_file, destination):
    """Create messages regarding the presence of the optional files, which will be 
    later added in html reports of Apps.

    Parameters
    ----------
    calibration_file: str or None
        Path to the '.dat' file with fine calibration coefficients. This file is machine/site-specific.
    cross_talk_file: str or None
        Path to the FIF file with cross-talk correction information.
    head_pos_file: str or None
        '.pos' file containing the info to perform movement compensation.
    destination: str or None
        The destination location for the head.

    Returns
    -------
    report_calibration_file: str
        message regarding the calibration file.
    report_cross_talk_file: str 
        message regarding the cross-talk file.
    report_head_pos_file: str 
        message regarding the head pos file.
    report_destination_file: str
        message regarding the destination file.
    """ 

    # Calibration file
    if calibration_file is None:
        report_calibration_file = 'No calibration file provided'
    else:
        report_calibration_file = 'Calibration file provided'   

    # Cross talk file
    if cross_talk_file is None:
        report_cross_talk_file = 'No cross-talk file provided'
    else:
        report_cross_talk_file = 'Cross-talk file provided'  

    # Head pos file
    if head_pos_file is None:
        report_head_pos_file = 'No headshape file provided'
    else:
        report_head_pos_file = 'Headshape file provided'

    # Destination
    if destination is None:
        report_destination_file = 'No destination file provided'
    else:
        report_destination_file = 'Destination file provided'

    return report_calibration_file, report_cross_talk_file, report_head_pos_file, report_destination_file