Any special circumstance you might be in. Don't feel obliged to write something that isn't necessary; this isn't one of those optional essays you find on the Common App.
This probably is a stupid question, but filling out the prelim doesn't obligate you to anything, does it? I'm interested but not really ready to talk to admissions or a BGO at this point.
You really are not obligated to anything until you're sworn in on I-Day. But, for the sake of other candidates who know what they want, don't stretch it that far before making a strong mental commitment.