When you buy the extension at usual price you automatically get the one time usage license.
You can use the same license on your dev / test / prod environment.
It enables you to use each individual product on a single website only, belonging to either you or your client.
You have to purchase the same package again if you plan to use the same extension in connection with another or other projects.
Contact us if you require it for more and we will quote a volume based price.