Extended license can be used for end products (web service or SAAS) that charges users for access or service(e.g: monthly subscription fee).
A single extended license can be used for a single end product and the end product can be used by you or your client. If you want to sell end product to multiple clients then you will need to purchase a separate extended license for each client. The same rule applies if you want to use the same end product on multiple domains(unique setup).
For more information on Themeforest’s licenses, you can check the below link