Here is the interesting part

1. Default runlevel. The runlevels used by RHS are:
2. 0 - halt (Do NOT set initdefault to this)
3. 1 - Single user mode
4. 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
5. 3 - Full multiuser mode
6. 4 - unused
7. 5 - X11
8. 6 - reboot (Do NOT set initdefault to this)

# id:3:initdefault:

. . .

note, that this is used by Redhat (and derivatives). debian use 0,1, and 6 from this, and have "2" as everything. 3,4 and 5 are unused.

Any unused runlevels can be tailored to your own needs.

See also inittab(5)