Commercial products (this does not apply to open source code) are conditioned upon acceptance of the licensing terms. Products are bound to the system where they have been activated though a unique system identifier named System ID, generated by ntop products.
As products can be activated on Virtual Machines that can migrate across physical hardware, this System ID may be subject to changes, breaking the license. Since July 21st 2021, the System ID has been strengthened to be migration-resistant.
However there are still cases where the System ID may change. For example when changing the Virtual Machine configuration or when moving the license from a physical (or virtual) system to another (e.g. your old system break down and you have replaced it with new hardware). In this case, you can migrate the license by generating a new one as specified in the How can I transfer a License to a new Server? section.