I actually didn't use that interpretation of the question at all. I used it as literally, "what was something that wasn't on your application?" -- something that makes you ridiculously unique but you just can't include it on your app somewhere...maybe like certain experiences that were important in your life, but you didn't want to write about it because you didn't want those experiences to define you as a candidate. I stuck with whatever was unique about me, but I couldn't include for whatever reason.
Good luck though, I'm sure you'll figure out something great to say--whether it was an experience, or it'd be a great chance to talk about something different you've done since you submitted your application as LBJ6 was recommending!