How to Unlock Subclasses
The first thing to do with Subclasses is obviously how to unlock them. Somewhat disappointingly, it requires completing the entire Single Player story to get to the point of unlocking them. After completing the Main Story, you will get a mission to scavenge another Memory Board.
This mission specifically comes after two “post game” missions, one of which requires you to dig at your base, and the other to rescue a survivor.
After which you will be directed to enter the Skill Trainer to look at your four new Subclass choices.