Once additional licenses are purchased in the online store, the additional licenses are added to the existing serial key. The administrator can now register additional users or the existing user can also register on different machines without uninstalling the product. Additional users can be on different machines (Windows 11/10/8/7).
The Additional licenses are for customers with desktop operating systems (Windows 11/10/8/7). If the user purchases an additional license, it means that one can register and use the product on different machines, not like in the case of an Add-on license, where the Add-on license grants the use of the product with different users on the same machine.
The steps of the registration are described in the Registering the Tiff Viewer Release version section.