1997.03.12 18:34 "Re: Unknown field in group 4 files", by David Babcock
Any tag with a value greater than 32768 (0x8000) is a privately defined tag. Sometimes these make it back into the TIFF spec, as PackBits did, but most often they do not.
I do not know whether Adobe makes available the list of tags that have been issued to thsoe requesting them, but if the file is of unknown origin, it would be hard to know whether the tag was used in accordance with what that list would say.
ADE Group, Caere Corporation