From GameWatcher: “Warhammer: Vermintide 2 does a lot of things right, but crafting isn’t one of them. One of the new additions to 2018’s sequel is the Illusion attributes to your weapons, which is in no way, shape, or form explained to you in the game.
Turns out Illusions are just a fancy roundabout way to say “weapon skins”, and we’re here to teach you what to do with them and where to put these Cosmetics.”
Source: N4G PC Warhammer: Vermintide 2 Cosmetics and Illusions Guide