What does Error Type refer to?

What does values of the filed 'Error Type' refer to in tbl_tiff_shadow and tbl_ocr_shadow?

FAILED_TO_CONVERT_PDF_TO_TIFF = 1,

FAILED_TO_CONVERT_IMAGE_TO_TIFF = 2,

FILE_NOT_ACCESSIBLE = 3,

UNKNOWN_ERROR = 4,

NUANCE_ERROR = 5,

FAILED_TO_OPEN_NATIVE_FILE = 6,

DISPLAY_ENGINE_UNAVAILABLE = 7

What are  ERROR TYPE-4, "UNKNOWN_ERROR"?

If there is an error/crashes during tiffings ( for  example pop-ups during native tiffing due to application we use etc. )  and if can not be categorized as any of the errors mentioned above , it should be marked as unknown .