These are the abominable intelligences the Adeptus Mechanicus warned you about, rangy paragons of martial prowess used to anchor the battle line or smash into the breach. With a 2+ save and a Preymark Crest that grants Precision when attacking enemies entrenched upon an objective, you can arm them either with heavy volkanite disintegrators or autoch-pattern bolters and plasma swords. There’s a Steeljack for every occasion.
The box contains three Steeljacks, with enough arms to outfit them all with either loadout. One can be built as a Theyn, while they can all also exchange their plasma swords for concussion gauntlets.