Correct in that it's the Tail Bag, however it is specifically designed to fit & be restrained on the passenger seat. It does not fit at all on the small carrier, now, if you modified the base & restraint system for it, then it could fit on there nicely, but it wouldn't fit on the rear seat anymore.
Just another oddity from Triumph, I'd of thought you could get differing attachment systems for the bag, but no, not that I've found anyway.