How do DoTs work

[Crusader]

I understand from the reply I received on Nerfing of Druid DoTs, that the patch made in 1999 set forth several requirements in assessing the damage done to monsters with DoTs. I am presuming the rules pertain to all classes/races. It seems the damage assessed is controlled by the 18 sec rule. There are three levels of damage...normal, additional, and penalized, however when looking on P 99 wiki for spell effects, and the spell damage in actual combat, it only shows two levels of damage...evidently the normal damage and the penalized damage (due to run/kiting for over 18 secs).
Is the damage assessed due to the type of spell cast on the mob first, root vs fear vs snare vs the DoT?
If root breaks and you then snare (root and snare do not stack) and run/kite for less than 18 secs, before the monster turns away due to low health, do you get normal damage or additional damage, or some of each. What if it takes more than 18 secs for the monster to turn away after run/kiting...is the damage done during the root or fear, DoT phase of the fight reduced?
And how is the "additional" damage acquired, and when does it take effect?

I am not posting this to "whine" about the DoT nerfing. I believe the answers to how the DoT damage is performed is essential to formulating a combat strategy using DoTs, especially for classes that depend on them.

Please do not simply repost the patch notes, since that does not answer the questions I am posing. Thank you for your help