Originally posted by M'Hael:

Back stab bonus is applied when attacking a creature that is unaware of you, you don't actually have to strike the back. There aren't any hitboxes with different damage multipliers, you either hit them or you don't.

This. To my understanding, There's no backstabbing, there's no headshots, there's no vulnerable hitbox locations on the enemy.

There is stealth attacks- Hitting an unaware enemy does more damage.
There is stagger damage- Hitting a staggering enemy does more damage.
There is resistance based damage- Hitting an enemy with the damage type it's weak to does more, with one it's strong to does less (EG: Pierce vs Skeletons does less, Blunt does more.)

And that's about it.