Theoretically, this item contains warning or error messages from Zabbix server. However, in practice, there has been no convention whatsoever which warning and error message get written to zabbix[log], so this item is essentially useless. Since the same functionality can be achieved by monitoring Zabbix server log file using log[/tmp/zabbix_server.log], item zabbix[log] should be removed in Zabbix 2.0.