Are you considered an official candidate if you are going to NASS?
Yes. Your candidate number is on your NASS acceptance letter.
Some good information here.
"If the information provided in your Preliminary Application indicates your record is strong enough, you will become an official candidate for admission and you will receive a candidate information letter as early as the July prior to your high school senior year. Upon completion of your candidate file, the Admissions Board will review your record."