2007.02.01 18:17 "RE: [Tiff] Interpreting SubIFDs tag", by Phil Harvey
Here is the document that invented the SubIFD concept -- this is the "Tiff Trees" approach. (http://partners.adobe.com/public/developer/en/tiff/TIFFPM6.pdf)
To quote this specification:
"If there is more than 1 child image for a given parent image, the NextIFD value of Child #1 must point to Child #2, and so on. The last Child’s NextIFD value must be zero."
I should point out that Adobe themselves don't store multiple SubIFD's like this.
The current version of the Adobe DNG converter writes multiple SubIFD's by storing a separate pointer for each in the SubIFD tag (and each SubIFD has a zero NextIFD pointer).
But this is definitely not the first time that Adobe has ignored their own specification.