2006.06.13 08:32 "Re: [Tiff] Editing a tiff image using libtiff", by Gerben Vos
An additional way to handle space is the introduction of an 'alloc-tag'. This tag indicates the size of the 'allocated' block in the file in contrast to the size tag which indicates the count of the bytes used.
There are already the FreeByteOffsets and FreeByteCounts (288 and 289) tags for keeping track of unused space. Nothing is specified about in which IFD they belong, though (the first? the last? wherever you want, and use all of them together?). The spec also says: "not recommended for general interchange", probably partly because of this, and also because it's not very nice to give someone a space-wasting file, I guess :-).