You're given a set of primary and secondary objectives to tackle, and you're free to go about completing these objectives in any way and in any order you see fit. (shortpause.com)
Credits are earned by completing objective - based waves during matches, and the higher the difficulty and the faster you achieve the objectives, the more money you earn. (escapistmagazine.com)
The challenges vary from; Be More Objective where players will have to race to complete all objectives while the rest of the player's team draws the enemy fire. (justpushstart.com)