The Tri bundle actually has two different HRM belts, not one additional one. The blue belt is a swim-specific belt, wheras the black one is tri-specific (i.e. you can also use this to gather swim data as well as all the run-specific info). Initially seems a bit pointless (why not just have the black belt), but I assume the blue one is made out of non-slip fabric and therefore doesn't have to be held in place with a wetsuit/tri-suit.

You get the blue heart rate strap which can be used for swim workouts. See the 'swim workouts' part of the description.