There are five different file formats you can output reports in: HTML, JSON, RTF, TXT or XML
Selecting the Open In Checkbox allows you to select which application will handle the report file.
System Default Application
This is the default application your macOS system uses to handle the selected report type. As an example if you select the HTML report the default application will be your default Web Browser application.
Selecting this radio button will enable the selection area.
Click in the selection area to bring up a Standard macOS Finder window allowing you to navigate to the application you wish to use to handle the exported Report file. The application you select must be able to actually handle the file type of the report you generate, otherwise it will display some form of error message. Some possible choices for apps would be Safari for HTML Reports or TextEdit for RTF or TXT Reports.
Show in Finder
If this box is checked, when you perform the Export Report action (i.e., select a report file format and click on the Report button), once the Report has been exported, your file will be revealed in a Standard macOS Finder Window. Unless you need to interact with the file immediately after export in the macOS Finder, this box should be left unchecked.
If this box is checked, your file will be closed in OmniMarkz after the Report file has been exported. By Default this box is left unchecked and your file remains open in OmniMarkz until you manually decide to close the file.