What happens if I set default system runlevel to 0 or 6?

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http://unix.stackexchange.com – What I understand from runlevels is that in each level some programs have permission to be executed and some don't. runlevel 3 boots system into CLI and runlevel 5 starts the default GUI. But I don't understand what do 0(shutdown) and 6(reboot) mean?(Start in shutdown/reboot mode?!). What happens If set initdefault to these values? (HowTos)