When you're using your Synology exclusively as a iSCSI target, and have assigned the complete disk space to your LUN's, you will get notifications about the volume being (almost) completely full. Obviously, in this case, you may not want to receive these notifications. Luckily, you can turn these off:
Note that this will only disable the notifications (e-mail, SMS, etc.). The widget and WebUI will still show the alerts.