Buddy killing. In the game if you play an "evil" character or you don't have enough meds then you have to kill your buddy. And when a buddy dies they are gone from that game forever.
Health stabalizing. When you are extremly low on health before you can use your meds you have you stabalize yourself. This is the one were you dig a bullet out of your leg with a knife.
Buddy rescue. In the game if you have a rescue ready buddy then whenever you go down your 2nd best buddy comes to your aid.