On Brainshell, a challenge is an exercice aimed at studying a specific subcategory such as Cryptography, Reverse Engineering, Forensiics and many more in the computer security field.
For example, in the Web Hacking category, each challenge focuses on a specific web applications vulnerability (SQL injections, XSS, CSRF, IDOR, etc...)
Players must find the one and only flag in a challenge to score the points.
Each challenge has the following flag format :
BS{flag} where 'flag' is the actual flag to submit