The Close Issue Checkbox is connected to some core functions inside SurveyTester. If you use a Classification Field for controlling the status of an issue, you can now also connect individual values of such a field with the close issue box. For example, you might have a status field with the items: New, Done, and Finished, where the selection of Finished would close the issue.
The connection is created inside the admin area, where classification fields are created.
Clicking on the Close Issue check box on the Field Values form will enable this feature.