Man, that’s the million-dollar question. Okay, so the Survey Corps—Scouts, whatever—they’ve got this rep for being suicidal maniacs with a high casualty rate, and on the surface, it’s kinda true. Their main gig is going outside the walls. That’s it. That’s the big one. But the ‘key missions’ thing shifts massively as the story goes on. Early on, it’s pure, desperate recon. They’re mapping territory, trying to find resources, maybe a safe place for humanity, all while getting eaten. It’s a death march with a purpose, I guess. The 57th Expedition in season 1, where they get slaughtered trying to reach that damn forest? Classic Scouts. Utter failure by normal standards, but they learned the Titans were heading for Eren, so maybe not.
Then it changes. Once they figure out Eren’s hardening power, suddenly the mission is retaking Wall Maria. That’s a concrete objective: plug the hole in Shiganshina. It goes from vague exploration to a military operation with a fixed goal. And later, well… let’s just say their mission parameters get blown wide open. Stopping the Titans becomes the smallest part of the job. Their final key mission is arguably something no one signed up for: dealing with the truth of their world and making an impossible choice for the fate of it.