I guess the Starting and Final Area fields define the Cone of the Effect. Which means the wave starts small and gets a bigger radius at the end within the enemies get damage.
The Max Damage is kinda tricky, I don't know the correct info either.. but i guess it's the summ of damage given. means if you choose 300 damage on the first fields and enter 3000 damage as max damage, then the spell will only deal 3000 damage at all. which means you could hit 10 units with 300 damage, if there are 15 units, then still only 10 units will receive 300 to reach the 3000. The damage sharing between the units could be different but i guess it's just the limit for given damage. if you want to always hit every unit in range with the skill, choose a pretty high max damage value..
correct me if i'm wrong ^^