The Export Zoomify Tiles command allows the user to export any loaded data to JPG files tiled in the configuration required by the Zoomify viewer. This command will create both the image tiles and the XML file required for Zoomify to use the data. You will also need the Zoomify plugin to allow viewing the data. Global Mapper is not allowed to distribute that, so you will have to obtain it from Zoomify yourself.
When you select the command, the Zoomify Export Options dialog (pictured below) appears allowing you to setup the export. This dialog allows you to specify the sample spacing and some additional options. There is also an Export Bounds panel which allows the user to set up the portion of the loaded data they wish to export.