When is nlobjError actually used??

From the help:

nlapiCreateError(code, details, suppressNotification)
Creates an nlobjError (complete with stacktrace) that can be thrown to abort script execution.

This code:

 function SS_OnBeforeLoad(type)  {      //  Test nlobjError and the stacktrace property      try      {          a();      }      catch(e)      {          var msg = new String();          if(e instanceof nlobjError)          {              msg += 'nlobjError Exceptionn';              msg += 'Code: '       + e.getCode()       + 'n';              msg += 'Details: '    + e.getDetails()    + 'n';              msg += 'UserEvent: '  + e.getUserEvent()  + 'n';              msg += 'InternalId: ' + e.getInternalId() + 'n';                            var stacktrace = e.getStackTrace();              for ( var i = 0; stacktrace != null && i < stacktrace.length; i++ )              {                  if ( i == 0 )                  {                      msg += 'Stack Trace' + 'n';                  }                                    msg += stacktrace[i] +