Challenger was conceived, developed, and launched for the sole purpose of testing different exploration maneuvers; conducting its performance was evaluated, configuring, and re-engineered for future exploration. Precautionary and safety was the Challengers main objective. the only reason that the launch is considered as a category two (2) decision was due to its beneficial properties regardless of loss; "uncertainty with regard to the outcome."
Category II is (a.) non-routine , (b.) non-recurring, and (c.) uncertainty with regard to the outcome. Considering the Challenger's tragidy was its third mission, this excludes a and b, leaving c as our result.