This documentation will help you to understand the differences between iViewer Licenses and LaunchPad credits.

iViewer 5 (and iViewer 4) can be licensed either via the legacy system or via LaunchPad. Both systems will take a Device ID and return you a license code to add to your guiDesigner project.

Once a device is licensed for iViewer, it can be transferred up to 3 times when devices are upgraded/replaced.

Any device that is licensed for iViewer can also be allocated to LaunchPad projects using the LaunchPad app using that existing iViewer license, without using another user credit.
This can only work if you have installed iViewer already and licensed it before you install LaunchPad app on the device. Otherwise, if you delete the iViewer app before installing LaunchPad, the Device ID will be different and the existing license will not work.