What exactly is FiTsZ? (Spoiler present)

Adampk said:
So what is exactly FiTsZ?
According to nasa, it was the future possibility even seven couldn't predict i.e. moon children choice.

But then in the ending, it was hinted that seven planned everything all along. 4 things point to that.
-One is the conversation konoha had with touya at the end that comet were not big enough to kill 1/3rd of population and seven was decieving UN 2
-Another is what nasa said. There is only one person who would dies today. Which actually turned out true, because she died and konoha survived. Though we were shown later her counter message to konoha that she will part ways with toayo. But technically it did happen because toaya and konoha are moved to there own lives and are not living together anymore in space station. It doesnt necessarily have to mean konoha death to be exact.
-Also there are minor fact that second seven death and his debries/dust was the reason why climate change problem improved. And that problem is the main reason seven came to a conclusion that 1/3rd of the population needs to reduced. Further strengthening that seven may have planned it.
-also according to some statistics, due to people migrating to space earth population will be reduced by 36.79% by 50 years which is almost 1/3 population reduction

So back to the question what exactly FiTsZ.
A keyword seven left behind to confuse the children to come to the choice he want them to take?

Well, that is not the only part of the question. What exactly "word" itself and why is it spelled like FiTsZ? Is it some sort of acronym, refrence or code word etc? Personality, its hard for me to believe its just some random word. Or am i missing something or interpreting something wrong?

Fortress_Maximus said:

If you want the literal answer (spoiler at your own risk)