(Koenma is explaining to the Spirit Detectives why they have to stop the hole to the Makai from growing. It will, he explains, allow very powerful youkai to get into the human world.)

p. 181
Koenma:
   B and C Class youkai are already able to go in and out of the Ningenkai.
   B Class youkai are at the Toguro level.

Yuusuke:
   B Class!?
   Are you saying Toguro is just a B Class youkai?

Koenma:
   What's more, deep in the Makai, there are youkai who are A Class--and S Class, which is beyond A.......!!
   Even Reikai doesn't know what to do with S Class youkai.

p. 182
Koenma:
   The Makai is eternally deep and vast.
   What Reikai controls is only a tiny part of it.

Botan:
   What....
   This is the first time I've heard this.

Koenma:
   What we can control is only half of the basement.
   The Makai is like an incredibly huge and deep underground building.

(Koenma pushes a button)

FX:
   pi

(The screen shows a schematic of the Makai. The area controlled by Reikai is very small.)

Screen caption:
   Only this part.

Koenma:
   From here on, it's too dangerous to step in.
   We can only expand our territory little by little over a long period of time.
   The space between the Makai and the Ningenkai is sealed by a strong barrier...
   But it's only to keep A and S Class youkai from leaving the Makai.

p. 183
Koenma:
   This huge barrier allows all youkai who are B Class or lower to pass through it.....
   No matter what, we must not let even one A or S Class youkai enter the Ningenkai!!

Yuusuke:
   Are you saying they're th... that dangerous?
   One A Class youkai alone is more dangerous than all.... the B Class youkai together?

Kuwabara:
   What's A or B?.
   You should explain it to me too. I can't see the screen.

Koenma:
   We have to stop it!!
   Before the hole becomes any bigger....!!

You might also want to read What Class is Hiei-chan, which parodies this very section of the manga, and answers the Burning Question 'What Class is Lively Little Hiei-chan?' ^_^