Each license is only valid for a single PC so if you want to use the same license type at more than one PC, you'll need to purchase more than one license (one license = one PC).
So, once you have a proper key, you can register it and use functionality, provided by the purchased license. Each license is activated with a special key. Other licenses are grouped by game/platform, they work with. Those from the General category affect the general program behaviour and work with every available file type. All licenses are divided into categories. RED MC allows you to choose a set of Licenses, which suits you most. You can watch the following video, which covers all the aspects of purchasing and registering the product: Thank you for your interest in the RED Modding Center project!