Types of License
Each plugin comes with a dedicated license. You will see the option to choose License on the pricing page, purchase sites for plugin PRO versions, as well as in Pricing and Ordering FAQ. The license can be one of 2 types depending on a number of sites it will be used on. The "number of sites" limit assigns the number of possible "production server" or "live" installations.
The single-site license allows for one "production server" installation.
You can still have multiple "testing" installations - for evaluating and testing purposes. For instance, your developers can install the plugin onto their local machines without any limits. However, only one "production" installation is allowed with "single-site" license.
The "multi-site" license allows an unlimited number of installations within one company. For every additional company, you have to purchase a separate license, despite the number of sites you are allowed to use with your current license.