When this value is set to a non-null-value it's value is used as php's error_reporting level inside of display() and fetch().

Smarty 3.1.2 introduced the muteExpectedErrors() function. Calling Smarty::muteExpectedErrors(); after setting up custom error handling will ensure that warnings and notices (deliberately) produced by Smarty will not be passed to other custom error handlers. If your error logs are filling up with warnings regarding filemtime() or unlink() calls, please enable Smarty's error muting.

See also debugging and troubleshooting.