How does the resistance system work in Dragon Age 2 work ?

So I upgraded Arcane shield 3 times. It gives 20 defence 20 all resistance to all party members.


Then I enchanted fire rune to Fenris's armor and it got 1700 fire resistance.





So is 20 any helpful figure when there is 1700 ? Does 1700 mean I will get 0 damage when enemies use fire on me ?





And how do I know which enemy has what resistance ? Do I just have to shoot and guess ?|||1700 fire resistance means fire damage will be reduced by 1700 for your party. When attacking enemies you pretty much do have to shoot to find their resistances