Some clarification:

On Friday, 03 dec 1999, Giles Lean wrote:

>     (iii) nobody agrees about _which_ runlevels should be available,
>           and several are taken and are not available
> 
> 	  0           shutdown
> 	  1/S         single user mode	      

Levels 1 and s/S define distinct states.  The former has file systems
required for multi-user operations mounted; the latter unmounts them.  You
would use the "s" (or "S") state is recommended for system software
installation/updates on HP-UX and, if I recall correctly, on SCO.

> 	  2	      former default multiuser (HP-UX 9.x, SCO?)

HP-UX 8.x perhaps; 9.x uses this as "multi-user without X".

> 	  3	      current HP-UX default runlevel

For 9.x and later, this is the "multi-user with X" state.

> 	  4
> 	  5	      ?? reserved by Solaris for something (or 
> is this 4?)

Level 5 is "shut down and power off the system."

> 	  6	      reboot (some OSes only, definitely SCO)

Same for HP-UX 10.x and Solaris 2.5.

--scott