Symbolism, Imagery, Allegory
How's this one for a symbol within a symbol? The lye kiss that Tyler lays on our narrator—and eventually everyone who's inducted into Project Mayhem—is an external mark of an internal condition: the desire to destroy.
Something else to consider: Fight Club has a thing about fairy tales. Did you notice it mentions both Snow White (7.43) and Cinderella (3.68)? Well, what about the princess it doesn't mention by name? You know, the one who was woken up by a kiss.
Think about Tyler Durden in that light: he's a prince charming waking up numerous (bruised, black-eyed, and male) sleeping beauties from their life-long sleepwalk to see the true nature of the world.
Maybe these guys are in touch with their feminine side after all.