Smarts 10.1.X, 24.3.X
Every Smarts error message has an associated severity. If you look at a message in the log file, you'll see something of the form "CI-F-something". The -F- is the severity - in this case, Fatal. The possible values, from most to least severe are:
Fatal, Error, Warning, Normal, and Debug.
By default, errors of level Warning and above go to the log file, and those of level Error or above are sent to the system logger. These defaults can be changed using the --errlevel and --loglevel startup options.
Either option can use a "level" of None to disable logging completely.