see this thread. i do not know if it's true but if it is it seems investors were not putting their money into a typical CD. the jist of this article is you were guaranteed to get principal back but the interest return if any would be dependent on the appreciation of the underlying assets such as gold, foreign currencies, etc.
https://www.bestcashcow.com/avoid-the-tiaa-4-year-diversified-assets-marketsafe-cd.html
