If you already have licenses in your system and you want to choose which ones are allowed to use a specific product, follow these steps:
Select in the list the product for which you want to condigure usage permissions. Then click on the "Set Product permissions" button (You can also select multiple products to set permissions in batch).
Select which licenses you want to grant permssions for the product in the list on the left, and drag them to the list on the right. Then click "Apply" to set permissions.
When someone tries to use your product, he will be able to activate it using one of these licenses.