It's fairly well known that Elves like to bring lots of cloth to trade along with their usual assortment of trade goods, and that said bins of cloth are always loaded first (to the point that the pack animals back in 40d would be easily overloaded, preventing any other trade goods from being brought). Of course, this has yet to be a problem in version 0.31.xx due to the disproportionately high trade capacities of pack animals. However, something very strange just happened in my 0.31.16 fortress...
For as long as I can remember, the Elves have been bringing lots of bins of cloth - anywhere from 10 to 20 bins in each caravan. I normally keep my cloth stockpiles fairly low (since cloth will wear out over time, even just sitting in bins), so I never bought any. Last Autumn, great fortune led a giant cave spider from the caverns into one of my cage traps and I set up a silk farm which has thus far been very productive, filling my stockpiles with over two hundred pieces of giant cave spider silk cloth. When the Elven caravan arrived the following Spring, I was in for quite a surprise: they brought enough trade goods to load up 5 horses, but out of those trade goods there were zero bins of cloth.
Maybe they were only bringing cloth in the first place because I wasn't producing any of my own? Maybe they brought it as emergency supplies similar to how caravans bring wood logs when you don't have any? Once, as an experiment, I forbade every single item in my fortress just before the Elven caravan arrived, and they managed to bring over 50 bins of cloth along with massive piles of food and wood - for all I know, the tendency for elves to bring cloth may be solely due to most fortresses never bothering to start a cloth industry and only keeping a few bins on hand in case of strange moods.
More research is definitely in order.