Google hits 1-4 use van Velsen documentation which acknowledges but doesn't process these chunks. Hit 5 is this thread, hit 6 is a Gamedev thread no-one got back to on the subject.
Weird, hits 2 and 3 for me were the 3DS format documentation (although I admit having to use Ctrl+F to find the chunks, the documents are so huge that it's extremely easy to miss them). Maybe it gave you different results though. Also I mentioned how rotation works because neither document seemed to explain it (one post in another of the hits explained it instead).
Good that was enough though =)
Well it's just that now I know that these chunks contain what isn't subchunks, + this link was in one of the ones from that let me google that for you search:
And that's a friggin beast of a find, so I thank you sir!
Edit: and the rotate info too