Represents number of mail messages assigned to operator. These includes
<ul>
    <li>0 - Unresponded
        <ul>
            <li>New messages received by system from visitors is set to this status. They remain in this status untill operator. [RESPONSE_UNRESPONDED]
                <ul>
                    <li>Operator replies to message by writing response. In that scenario message changes status to (3) [RESPONSE_NORMAL]</li>
                    <li>Operator replies to message by choosing No reply required option. In that scenario message changes status to (1) [RESPONSE_NOT_REQUIRED]</li>
                </ul>
            </li>
        </ul>
    </li>
    <li>
        (1) - No reply required
        <ul>
            <li>Sometimes visitor replies to our e-mail where visitor response does not require any feedback from us. In that scenario operator marks message as no reply required. So visitor message get's this status. [RESPONSE_NOT_REQUIRED]</li>
        </ul>
    </li>
    <li>(2) - We have sent this message as reply or forward
        <ul>
            <li>Messages operator has sent is set with this status. This status indicates it was an operator send message. [RESPONSE_INTERNAL]</li>
        </ul>
    </li>
    <li>(3) - Responded by e-mail
        <ul>
            <li>As soon operator writes response to visitor e-mail visitor message changes status to this to indicate it was responded. [RESPONSE_NORMAL]</li>
        </ul>
    </li>
</ul>

This statistic is based on mail messages and their status. Not on a ticket itself. Each ticket consist of more than one mail message.