Challenge codes are used for linking workouts or challenges with the same score type to accumulate over a period of time.

Example: You're running a 10 day cash-out push-up challenge - you can link the workouts/challenges to accumulate the amount of push-ups done so it gives you one big total at the end. It's also easy to see the history and view your scores over the said time. You would input the same challenge code in each workout to link the scores - i.e. Nov24Pushups