This page is displayed from the File>Export tool, but only if you are exporting multiple layers (or faces) and the Prompt for layers to export option has been selected.
Use the check-boxes to select the layers and faces to export. You can also change the order they are exported using drag and drop or the right-click context menu.
The Face column is only available if you have chosen to export both faces from the Import Export Settings Non-native page.
Export all selected layers to a single file
This option is only available for file formats that support this feature. Presently Adobe PDF is the only format that does support it. If you switch the option off then a separate file will be created for each layer/face.