CodenameOne supports generating a "preview MIDlet" on the fly to allow designers to preview their UI on an actual device. This is an immensly powerful tool for a designer providing the designer with the "feel" of the theme on a specific device.
While CodenameOne has support for running directly on a device for some specific devices (Nokia, RIM), there is still no standard for installing MIDP applications directly from the PC. This operation allows you to generate a jad/jar pair which can be installed manually on most common devices in the market by saving them to the SDCARD and clicking the jad or jar to install/run.
A few devices will only work when running in OTA (Over The Air) mode, which effectively means you would need to host the jad/jar on an appropriate server and install them via a device browser.
When a UI is executed on the device a combo box is presented allowing you to pick the UI form to start with. This is useful for a designer to preview starting from an arbitrary form in the middle of the application and not necessarily going through the full application flow which might not be applicable without application code.