SQ42 is just Star Citizen with a singleplayer campaign. Rather than being able to run around the galaxy and do whatever, you're instead going to be strapped into a linear storyline. You'll be a UEE fighter pilot. From the footage we've seen thus far, the missions will have a formula where
you wake up on the Idris (your mothership), go to the mission briefing (NPCs standing next to a powerpoint presentation), go to the hangar, get in your ship, and then go off and do the mission.
One of the missions shown thus far has you flying to a space station, getting out of your ship, and infiltrating the station for a stealth/FPS segment, where you try to reach a room (you can either do so by stealthing or running around shooting everyone). Presumably once you have completed your mission objective, you will fly back to the Idris, land, and the level will end. Rinse and repeat a dozen times and that's your campaign.
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This should be simple, scripted stuff, no different from a Call of Duty campaign. It's funny how in the 9 years since Star Citizen was successfully kickstarted, 7 Call of Duty campaigns have been released, and yet CIG has yet to release a single SQ42 level.