And here I was going to answer:
NO. That information is secret!
Some runlevels are specific to system shutdown or special states, but the available ones can specify a different set of programs, processes, or services to run. Boot to runlevel 2 and you have a web server and router sans desktop, runlevel 3 you have an XWindows desktop with XFCE and media players in your menu, runlevel 5 and you find yourself running GNOME with OpenOffice and business applications in your menus: all without multi-os boot, virtualization, or jumping through hoops -- just the magic of Unix style runlevels!
And some people want to retire that system. Are they Crazy? [NOTE: rhetorical question, not to be answered rudely please.
And after all that: What made you think that they are all "required"?
Your reading (See post by Acid*) will teach you which are required under system that use runlevels.