Code Name Verity refers to Julie Beaufort-Stuart's code name, Verity. We know—that's pretty obvious. However, why might the author have decided that Julie's code name is important enough to get top billing on the cover of the book?
First, as we've already discussed, this is really Julie's story. Second, Julie's code name is extremely important for what it tells us about her—verity, or truth, is important to her. She lies her head off to the Ormaie Gestapo, but as she and Maddie both comment, her story is true overall. It's the last thing she says, over and over, as she's waiting for execution.