So now we do have an answer from Fearlord, the HC infernal who was kind enough to check on what "unused" means:
The answer is: Any card from a faction's whole pool of cards costing up to three and not granting victory.
Which means: Forget what I said about detachment choices. Just pick which you want most for your strategy. Good god, play Cygnar R/G/B with all the Journeymen readily available...
Game Report is delayed, btw. Still trying to schedule it. Hang in there!
And until then, farewell.