descr_projectile dmg and EDU dmg. Do they get added?

[Lonck]

I noticed that in descr_projectile each projectile has it's own dmg value and I also noticed that EDU unit also has a dmg value. Do these dmg values get added or does the game takes the highest dmg value or what?
Also can anyone explain descr_area_effects.xml or has a tut somewhere? I'm gonna start testing on my own but an expert answer would be better. What I don't understand is what these mean

         <impulse_radius>4.5</impulse_radius>         
         <impulse_force>425.0</impulse_force>
         <effective_radius>2.0</effective_radius>
         <effective_damage>1.0</effective_damage>
         <kill_radius>1.0</kill_radius>
         <kill_damage>200.0</kill_damage>

and their numbers.

[Lonck]