Our themes sale are managed by Envato and according to their policies each theme/license can be used for one product (site), read more about Themeforest license standards.
You can use it on multiple test sites but if you want to use it on multiple sites/domains(end product), you have to buy a license for each project.
So you can just use your one license for one domain; not using it for multiple domains, not changing it between several domains. But you may change your domain completely and want to register Publisher with new domain permanently. Or you use Publisher on test domain or developer area and now you are migrating to main website. In these cases, you can change your registered domain without limitation. The important point is, not use one license with several domains simultaneously.
If you need more details on this you could ask Envato help center.
How to Change Licensed Domain
For this purpose, sign into your account in BetterStudio community first. Now you see a button at the top of community page, LICENCE MANAGEMENT button.
Click on it
If you bought a new license, you can add a new registration to this account. If you already registered your license and want to change domain, first delete the existing registration then add a new one with new domain name.