Like TobiSGD said, runlevels do not represent magic numbers. It is a way for the system to operate in a certain state, like single-user, graphic, console etc. It is a de facto standard to use 1 for single user, and 6 for shutdown. But by no means there is something in the kernel or anywhere else that makes this obligatory. Different distros apparently had different preferences. Debian uses 2 for graphical operation (try to figure this out when you are Red Hat educated!) If a runlevel is not used, it is not used.