Flat Damage on skills and Devotion

i still cant understand when and where flat dmg # are applied.
for example : will of ratosh has a RR mark for 8 sec … thats clear so far …
it also has flat vit and aether … what happens with that ?
and there is spectral binding and spectral wrath for the necro.
SP has flat aether dmg for my weapon dmg i guess
SW has flat vit and aether but that is just the actuall damage i do when enemys strike me?