Hi Baek007,
Here are some examples of the type of log messages you see at the different levels. It is up to the developers of the code to determine what the best level for a specific log would be.
The SOL you references describes it pretty good...
emerg - Emergency system panic messags - examples below show that system services are restarting or a service cannot start or that there is a hardware issue.
alert - Serious errors that require administrator intervention - examples show that an administrator might need to restart services.
crit - Critical errors, including hardware and filesystem failures - examples show devices having disk errors and the smartd process is producing the crit errors.
Jan 1 15:42:00 hostname emerg system_check[23386]: 010d0009:0: Mezzanine +2.5V voltage (Slot 1): voltage (5499) is too high
Jan 1 18:45:56 hostname emerg logger: Re-starting tmm
Jan 1 11:56:43 hostname emerg logger: tomcat can't start because /var is at 100%.
Jan 1 14:49:52 hostname emerg logger: Re-starting tamd
Jan 1 14:49:52 hostname emerg logger: Re-starting tmrouted
Jan 1 03:57:47 hostname emerg logger: Re-starting mcpd
Jan 1 01:38:57 hostname alert logger: Bigstart killing service stpd
Jan 1 01:38:57 hostname alert logger: Bigstart killing service tmrouted
Jan 1 01:39:26 hostname alert logger: Bigstart killing service chmand
Jan 1 11:53:00 hostname crit smartd[4864]: Device: /dev/sdb, 1 Currently unreadable (pending) sectors
Jan 1 10:43:18 hostname crit smartd[3509]: Device: /dev/hdc, 4 Currently unreadable (pending) sectors
Jan 1 03:51:49 hostname crit smartd[3455]: Device: /dev/sda, FAILED SMART self-check. BACK UP DATA NOW!
Hope this helps,
Seth