Questions About Deathfire Touch

Your damaging abilities cause enemy champions to take damage equal to 8 + 50% of your bonus attack damage and 20% of your ability power, over 4 seconds

Deathfire Touch has reduced effectiveness when applied by area of effect or damage over time abilities.

• Area of Effect: 2 second duration.
• Damage over time: 1 second duration.

Before I question, it should be noted that in-game the damage is 6 + 50% bonus attack damage and 20% ability power.

As for my question - Let's say your Deathfire Touch does 20 damage, over 4 seconds. I interpret this as 5 damage per second. 5 * 4 = 20

Area of effect duration is 2 seconds. Does that mean it does 10 damage? Damage over time is 1 second. Does that mean it does 5 damage?

Or is it 20 damage per second, so after 4 seconds it does 80 damage, area of effect is 40 damage and damage over time is 20 damage?

Would really appreciate it if somebody could clear this up for me.

Then their are champions like who have skill shot spells that can hit 2 targets, is that considered "single target" or "area of effect"