​I don't understand block/parry, with some characters, I see blue circle, I block, do not take damage, enemy is stunned, I shoot head. With other classes I see blue circle, I block, the character blocks, but it takes damage and the enemy is not stunned, what is the difference?, I block and the animation shows that the enemy weapon bounces and is the same animation of other classes when it does not take any damage, but in other classes it does take damage and enemy is not stunned bulwark and tactical specially (i'll update if i discover other class)

In addition to what was mentioned above, if you use a Fencing or Balance weapon, you can perform a Perfect Parry on an incoming attack (except for red ones). This will block the damage, stagger the enemy, and allow you to execute a Gun Strike (a shot to the head), which can restore 1 segment of armor . 

With Block weapons, you can perform a Perfect Block on an attack. While this won't stagger the enemy, it will block the damage and grant you one charge. Once you accumulate two charges, your next melee attack will have an AOE effect (a big explosion) and will also restore 1 segment of armor.