RunLevels.md

Boot-Level Mode Action

0 	System halted
S 	Single-user mode, booted to system console only, with only root filesystem mounted (as read-only)
s 	Single-user mode, identical to S except the current terminal acts as the system console
1 	Single-user mode with local filesystems mounted (read-write)
2 	Multi-user mode with most daemons started and Common Desktop Environment launched
3 	Identical to runlevel 2 with NFS exported
4 	Multi-user mode with VUE started instead of CDE
5 	Not used/User-definable
6 	Not used/User-definable

0 – System halt i.e the system can be safely powered off with no activity.
1 – Single user mode.
2 – Multiple user mode with no NFS(network file system).
3 – Multiple user mode under the command line interface and not under the graphical user interface.
4 – User-definable.
5 – Multiple user mode under GUI (graphical user interface) and this is the standard runlevel for most of the LINUX based systems.
6 – Reboot which is used to restart the system

runlevel0.target, poweroff.target – Halt
runlevel1.target, rescue.target – Single-user text mode
runlevel2.target, multi-user.target – Not used (user-definable)
runlevel3.target, multi-user.target – Full multi-user text mode
runlevel4.target, multi-user.target – Not used (user-definable)
runlevel5.target, graphical.target – Full multi-user graphical mode (with an X-based login screen)
runlevel6.target, reboot.target – Reboot

Distributed Application launch step

runlevel0.application, init application
runlevel1.application, Read and initialize internal properties
runlevel2.application, connect to others applications
runlevel3.application, exchange data with others application and updates theirs internals
runlevel4.application, Application is in stanby 
runlevel5.application, Application reset  and goes to runlevel0.application
runlevel6.application, Application shutdown after saving necessary data