Hi,
There are two types of licenses here.
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Regular license which allows you to use the theme or plugin which you purchased in only one project (which means only on a single domain). If you want to use that same theme or plugin on a different domain, you need to purchase a regular license again for both. You can customize the look of it or modify the functionality or even extend it to perform exactly the way you want for your business or your client projects. You cannot resell them saying that they are your own plugin or theme.
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Extended license is a special kind of license which is a bit confusing. You need this license only when you are going to charge an end user (which means your client) for using your services. For eg: If you are going to run a dating website and going to charge your users on monthly basis for memberships, in that case you will need extended license of the theme or plugin which you decide to use on your dating site.
Hope that helps. And good luck!