Because I'm lazy, I'll quote myself:

I can't find you that exact post, but I'll give you a quick summary of the concept:
(arbitrary numbers, the concept is the same)
Suppose:
Tomwarrior tanking Bobdragon gains 5000 threat per 10 seconds. Every 10 seconds, Bobthedragon uses Knock Away, which drops Tomwarrior's threat by 50%.
At the 10 second mark, Tomwarrior has gained 5000 threat and lost 2500, so he is now at 2500 threat, a total increase of 2500 during those 10 seconds.
At the 20 second mark he has gained another 5000 threat, bringing him to 7500 before the knockback. The knockback hits, and places him at 3750 threat. That's a total increase of 1250 threat over those 10 seconds.
At the 30 second mark he has gained another 5000 threat, bringing him to 8750 before the knockback. The knockback hits, and places him at 4375 threat. That's a total increase of 625 threat over those 10 seconds.
Continue this experiment long enough, and Tomwarrior's threat per 10 seconds approaches 0. That is what is commonly referred to as the threat/aggro plateau.
Practical consequence:
On mobs with percentage based de-aggro effects, you need to:
a) Increase the amount of tanks, lenghtening the time between knockbacks per tank, giving you more time before the aggro plateau is reached.
B) Kill the mob before the aggro plateau becomes an issue.
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There is no aggro ceiling as in "there is no way a tank can get more aggro than this" unless the mob has a percentage based deaggro effect. In theory, if Knock Away misses, the tank will temporarily be able to get above the aggro ceiling by a tiny amount, but will drop back down to it. In reality, you will likely never see the tank be infinitely close to the aggro ceiling, since the threat gained per time unit will be so low for long before that, that dps/healers will draw aggro with their linear threat curve.
Fade does not lower permanent aggro, and does not give you lower threat generation while it's active. It simply lowers your threat by a fixed number for the duration, and gives it back when it drops.