The CallStack settings in TestComplete allows the user to track the function call that has been made to reach the particular log message.
By default this settings is enabled fonly for log.error message.
I guess this helps the testers to debug the code , whenever it fails...
For More info please visit
http://www.automatedqa.com/support/viewarticle.aspx?aid=11359&SearchQuery=call%20stack
Happy Testing & Debugging...