In Metal Gear Survive, you belong to the survivor class and you can upgrade your skills with the in-game currency KUB. Later in the game you will be able to unlock more classes. However, to level them and use to their unique abilities, you have to unlock the subclasses first. To unlock them, you have to play through the main story of the single player campaign.
After completing all chapters you’ll get a milestone mission where you have to help Virgil AT-9 to locate a memory board. Here you will encounter several opponents but if you have killed them and collected all objects, you will be rewarded with four new sub-classes: Assault, Jaeger, Medic and Scout.
You won’t get this mission until you have completed two more missions after the actual end. In the first, you dig in your base and the second mission requires you to save another survivor.
So keep some KUB for the subclasses. You can level them individually.
The Assault class likes close combat and is always stay close to the enemy. Some skills make this class the perfect leader and even give bonuses to the other team members.
The Jaeger is a silent killer with the ability to shoot several arrows with his bow. In addition, this subclass can lie down far away from the opponent. His abilities are designed to attack and target from afar. Choose this sub class if you want to play with a sniper rifle or bow.
If you play alone, you should think twice before picking this subclass. For example: you can stop stamina drop for a short time, or backstab enemies, so sneakily attack from behind. If you perform such a kill, you will even recover some health if you unlock the appropriate ability. But as a Medic, you also have some skills that can brutally knock enemies down.
This subclass is especially good for players who don’t like to play in the group. With a skill, the scout can even make invisible and so sneak past opponents. Also, your traps and other units get stronger and you can repair them much easier and faster. The Stab and Grab ability gives you more Kuban energy when you make a sneak attack from behind. You also collect resources faster.