Once you have purchased additional licenses in the online store, the additional servers will be added to your existing serial key. The Administrator can register the product on that many more server operating systems.
If the user purchase an additional license, it means that he can register and use the products on different machines, not like in the case of Add-on license, where the Add-on license grant to use the product with different users on the same machine.
The steps of the registration described in the Installation steps section.