About splogger's fac? 2? 3?

[ling]

Hi:

I've read the man page of splogger carefully.But there is still something I 
don't understand. 

         splogger [ tag [ fac ] ]
logs messages with facility fac.
fac (default: 2) must be numeric. 

What's the meaning of "fac"? Is it use in syslog?
Some time it is 2 (default),some time it is 3.
What the diffirent between 2 and 3 ? 

Thanks