“The Tree of Sephirot is basically a ranking chart. It has the ranking of the souls of God,
angels, and humans graded on a ten-step pyramid. Just think of it like that.”

“To put it bluntly, it’s like a map of God’s absolute reign that basically says that this
territory is for humans and this territory is for God—so don’t go coming in all willy-nilly.”

“The number of people and the number of angels are all decided beforehand, so
normally, a human cannot climb to the status of angel. It goes both ways; an angel also
cannot fall to the level of a human.”

“Basically, all the ranks are filled to capacity.”

“By the way,” Kanzaki continued after Tsuchimikado’s comment, “what we call Angel
Fall is referring to an angel of a heavenly rank being forced down to the rank of humans.
Like a cup that was already filled, if you pour in one more drop, if an angel falls to the
rank of humans, then—Is something the matter?”

[...]

“I’m not sure what you mean by that question. Anyway.” Kanzaki gave a cough. “Because
an angel has been forcibly moved from a higher sefira to a lower one via Angel Fall,
because of the fluctuations it has caused, the four worlds creating the form of the ten
sefira—specifically, Olam Atzilut (the shaping world), Olam Beriah (the creating world),
Olam Yetzira (the formative world), and Olam Asiyah (the physical world)—are being
affected”

“...Tsuchimikado-san, what on earth is this person saying?”

“Hmm, how would you put it simply, nya? Like you said, everyone’s insides and outsides
got swapped out. It’s essentially a game of musical chairs. At the start of the game, all
the chairs and people who need to sit are scattered about, right? But there isn’t a chair
for every single participant in the game. The one who gets bounced out would end up in
heaven, sitting on the angel’s chair.”

Substitution.

- Toaru Majutsu no Index Volume 4 Chapter 1 Part 1