As I known there is no classical init in ubuntu, instead of init ubuntu uses upstart. Upstart is capable of fully simulation of classical init.
The problem is I still don't understand how upstart known which runlevel to boot now. For example, when I run ubuntu desktop how it knows that it needs to run runlevel for X and when I run ubuntu server how it knows to run runlevel for Single-User Mode.
As I know there are different folders for each runlevel rc x.d, do you know what "rc" stands for?