Equipment with the designation ST are part of a special themed set, and when all items of the set are equipped together your character will transform into a larger sprite. Unlike regular untiered (UT) items, ST items can be purchased for Realm Gold through Mystery Boxes, in addition to dropping from enemies.
All special themed sets, except for the Oryx Awesome Set, confer an additional stat bonus if all four items of the set are equipped together.
Some UT items are part of a set through similar aesthetics but are not considered ST, because they do not transform the player to a larger sprite if the full set if equipped. Like other UTs, these cannot be purchased for Realm Gold.