2009.01.06 10:51 "Re: [Tiff] TIFF standards and technical notes", by Gerben Vos
Whatever you do, please disregard section 22 of the TIFF specification completely, and only use TIFF TechNote 2 (or apparently the Photoshop TTN). Section 22 is self-contradictory and incomplete. libtiff has reading and writing support for TTN2. After a lot of hard work and reverse engineering, Joris van Damme has implemented read-only support for TIFF variants created by the section 22 implementations that he could find (and many of these were indeed incompatible amongst themselves); this work has also been incorporated into libtiff, but it should only be used if you really, really, really have to. Please don't ever create new TIFF files based on section 22.
I don't know about section 21 or the details regarding YCbCr images, sorry.