Hello Grant . . .
Yes, ST is short for sodium thiosulfate.
As far as chloramines go, the ST breaks the weak ammonia/chlorine bond and neutralizes the chlorine -- then the bio-filter is left to dispose of the ammonia.
BTW, there's usually less of an ammonia spike from the chloramines with a routine water change than one would get from feeding their koi.
As for chloramines and activated carbon, I've been told that they will deal with them IF there's enough AC and IF the water's dwell time in the unit is sufficient. But, hey, I'm no expert on that point.
_____ Don